Resolve custom application stylesheet with Webpack (#3373)

This implementation is a bit smaller and still has the following benefits:

* No need of app/javascript/packs/custom.js
For custom stylesheet, it typically has only
"require('../styles/custom.scss')" and is redundant.

* No need to extract vendor stylesheet to another asset
Extracting vendor stylesheet could be forgotten by developers who do not
use custom stylesheet.

app/helpers/style_helper.rb

@@ -1,17 +0,0 @@
-# frozen_string_literal: true
-
-module StyleHelper
-  def stylesheet_for_layout
-    if asset_exist? 'custom.css'
-      'custom'
-    else
-      'application'
-    end
-  end
-
-  def asset_exist?(path)
-    true if Webpacker::Manifest.lookup(path)
-  rescue Webpacker::FileLoader::NotFoundError
-    false
-  end
-end