fix($core): style loss under build for package that specifies `sideEffects:...

fix($core):  style loss under build  for package that specifies `sideEffects: false` (fix #2350) (#2471)

packages/@vuepress/core/lib/node/webpack/createBaseConfig.js

@@ -230,7 +230,7 @@ module.exports = function createBaseConfig (context, isServer) {
         })
 
   function createCSSRule (lang, test, loader, options) {
-    const baseRule = config.module.rule(lang).test(test)
+    const baseRule = config.module.rule(lang).test(test).sideEffects(true)
     const modulesRule = baseRule.oneOf('modules').resourceQuery(/module/)
     const normalRule = baseRule.oneOf('normal')