This change swaps the CommonJS require() syntax in the Webpacker
application.js pack template file and in documentation examples with ES
module import syntax.

Benefits of this change include:

Provides continuity with the larger frontend community: Arguably, one of
the main draws in adopting Webpacker is its integration with Babel to
support ES module syntax. For a fresh Rails install with Webpacker, the
application.js file will be the first impression most Rails developers
have with webpack and Webpacker.  Most of the recent documentation and
examples they will find online for using other libraries will be based
on ES module syntax.

Reduces confusion: Developers commonly add ES imports to their
application.js pack, typically by following online examples, which means
mixing require() and import statements in a single file. This leads to
confusion and unnecessary friction about differences between require()
and import.

Embraces browser-friendliness: The ES module syntax forward-looking and
is meant to be supported in browsers. On the other hand, require()
syntax is synchronous by design and not browser-supported as CommonJS
originally was adopted in Node.js for server-side JavaScript. That
webpack supports require() syntax is merely a convenience.

Encourages best practices regarding optimization: webpack can statically
analyze ES modules and "tree-shake", i.e., strip out unused exports from
the final build (given certain conditions are met, including
`sideEffects: false` designation in package.json).

Restore attribute alias resolution for `attribute_{for_inspect,present?}`

It is almost no longer used in Active Record.

Backward compatibility to work `Marshal.load(legacy_relation.dump)`

Follow up to c07dff72.

Actually it is not the cop's fault, but we mistakenly use `^`, `$`, and
`\Z` in much places, the cop doesn't correct those conservatively.

I've checked all those usage and replaced all safe ones.

Follow up to fc0090b3.

It is considered as a regression for #34384.

Use bash code fences and prompts for shell code [ci skip]

Follow-up to #39594, which added CSS in order to select shell commands
sans prompts on triple-click.

This commit adds several bash code fences and prompts where they were
missing, and removes a few where they were inappropriate.

Use COMPONENT_ROOT as app_root when present in test unit reporting.

Use html-based edge badge instead of image

Easier shell command selection in Rails guide

Shell prompt is already unselectable
Co-authored-by: NJonathan Hefner <jonathan@hefner.pro>

Add a setting to specify that all string columns should be immutable

All types are supported for PostgreSQL >= 9.3.

ActiveRecord PostgreSQL adapter: parse quoted values in range output

A relation has a predicate builder, so if a predicate handler which is
referenced by a legacy predicate builder is removed in the new code,
`Marshal.load(legacy_relation.dump)` will fail due to referencing
undefined constant.

I've restored no-op `BaseHandler` constant alias to make cache rotation
easier during Rails 6.1.

Fixes #39601.

And benchmark with this branch for immutable string type:

```ruby
ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :name
    t.string :fast_name
  end
end

class User < ActiveRecord::Base
  attribute :fast_name, :immutable_string
end

user = User.new

Benchmark.ips do |x|
  x.report("user.name") do
    user.name = "foo"
    user.name_changed?
  end
  x.report("user.fast_name") do
    user.fast_name = "foo"
    user.fast_name_changed?
  end
end
```

```
Warming up --------------------------------------
           user.name    34.811k i/100ms
      user.fast_name    39.505k i/100ms
Calculating -------------------------------------
           user.name    343.864k (± 3.6%) i/s -      1.741M in   5.068576s
      user.fast_name    384.033k (± 2.7%) i/s -      1.936M in   5.044425s
```

Fix broken references to classic mode guide where possible and use new guide where appropriate (#39603) [ci skip]

Upgrade-safe URL-safe CSRF tokens

This allows applications to safely upgrade to Rails 6.1 without
breaking tokens while the deploy is still being rolled out.

[ci skip] 4.2 Release Notes - Update link to autoloading guide to classic mode guide inst…

In Rails 4.2 we introduced mutation detection, to remove the need to
call `attribute_will_change!` after modifying a field. One side effect
of that change was that we needed to enforce that the
`_before_type_cast` form of a value is a different object than the post
type cast value, if the value is mutable. That contract is really only
relevant for strings, but it meant we needed to dup them.

In Rails 5 we added the `ImmutableString` type, to allow people to opt
out of this duping in places where the memory usage was causing
problems, and they don't need to mutate that field.

This takes that a step further, and adds a class-level setting to
specify whether strings should be frozen by default or not. The default
value of this setting is `false` (strings are mutable), and I do not
plan on changing that.

While I think that immutable strings by default would be a reasonable
default for new applications, I do not think that we would be able to
document the value of this setting in a place that people will be
looking when they can't figure out why some string is frozen.
Realistically, the number of applications where this setting is relevant
is fairly small, so I don't think it would make sense to ever enable it
by default.

PostgreSQL uses nested quoting on output for values inside range or composite
types under certain conditions(*), which include the format for timestamps.
ActiveRecord::ConnectionAdapters::PostgreSQL::OID::Range doesn't handle this in
 #cast_value.

(*) If their string representation contains parentheses, commas, double quotes,
backslashes or whitespace (manual section 8.17.5, 8.16.6).

This results in a string containing nested quotes being passed to
AR::CA::PostgreSQL::OID::DateTime, which causes two problems:
 * The special-case handling for BCE dates is bypassed, resulting in incorrect
   positive instead of negative year values.
 * ActiveModel::Type::DateTime's fast_string_to_time parser can't be used and
   falls back to Date._parse

Note that this PR only implements the escaped range bound format when parsing
Postgres output, not when serializing values for Postgres. Postgres also
requires its input to be escaped in the same cases, with the exception of
whitespace. Fortunately, none of Postgres' pre-defined range types use any
characters requiring escaping other than whitespace. See Rails PR #39585 for the
serialization side.

Use statement cache for STI classes

Use indifferent access for config hash in actioncable postgresql test

Strict match when choosing cookie domain for host

Deprecate `map!` and `collect!` on `ActiveRecord::Result`

These actually does not inplace mutate result. Use true `map` instead.

Revise encrypted credentials section [ci skip]

Unify coverage of collection helpers [ci skip]