Add method to confirm rich text content existence by adding ? after content name

This change introduces a rich text object to make
it easier to confirm it context is existing or not.

If we have a class like below.

class Information < ApplicationRecord
  has_rich_text :notes
end

Before:
i = Information.new
i.notes? => NoMethodError
i.notes = "Some sample text"
i.notes.present? => true

After:
i = Information.new
i.notes? => false

i.notes = "Some sample text"
i.notes? => true

actiontext/CHANGELOG.md

+*   Add method to confirm rich text content existence by adding `?` after content name.
+
+    *Kyohei Toyoda*
+
 *   The `fill_in_rich_text_area` system test helper locates a Trix editor and fills it in with the given HTML:
 
     ```ruby

actiontext/lib/action_text/attribute.rb

@@ -13,6 +13,7 @@ module Attribute
       #   end
       #
       #   message = Message.create!(content: "<h1>Funny times!</h1>")
+      #   message.content? #=> true
       #   message.content.to_s # => "<h1>Funny times!</h1>"
       #   message.content.to_plain_text # => "Funny times!"
       #
@@ -29,6 +30,10 @@ def #{name}
             rich_text_#{name} || build_rich_text_#{name}
           end
 
+          def #{name}?
+            rich_text_#{name}.present?
+          end
+
           def #{name}=(body)
             self.#{name}.body = body
           end

actiontext/test/unit/model_test.rb

@@ -18,6 +18,7 @@ class ActionText::ModelTest < ActiveSupport::TestCase
     assert message.content.nil?
     assert message.content.blank?
     assert message.content.empty?
+    assert_not message.content?
     assert_not message.content.present?
   end
 
@@ -26,6 +27,7 @@ class ActionText::ModelTest < ActiveSupport::TestCase
     assert_not message.content.nil?
     assert message.content.blank?
     assert message.content.empty?
+    assert_not message.content?
     assert_not message.content.present?
   end