PERF: Update ember-auto-import and webpack (#15919)

This makes a small improvement to 'cold cache' ember-cli build times, and a large improvement to 'warm cache' build times

The ember-auto-import update means that vendor is now split into multiple files for efficiency. These are named `chunk.*`, and should be included immediately after the `vendor.js` file. This commit also updates the rails app to render script tags for these chunks.

This change was previously merged, and caused memory-related errors on RAM-constrained machines. This was because Webpack 5 switches from multiple worker processes to a single multi-threaded process. This meant that it was hitting node's default heap size limit (~500mb on a 1GB RAM server). Discourse's standard install procedure recommends adding 2GB swap to 1GB-RAM machines, so we can afford to override's Node's default via the `--max-old-space-size` flag.

app/helpers/application_helper.rb

@@ -135,6 +135,22 @@ module ApplicationHelper
     path
   end
 
+  def preload_vendor_scripts
+    scripts = ["vendor"]
+
+    if ENV["EMBER_CLI_PROD_ASSETS"] == "1"
+      @@vendor_chunks ||= begin
+        all_assets = ActionController::Base.helpers.assets_manifest.assets
+        all_assets.keys.filter_map { |name| name[/\A(chunk\..*)\.js\z/, 1] }
+      end
+      scripts.push(*@@vendor_chunks)
+    end
+
+    scripts.map do |name|
+      preload_script(name)
+    end.join("\n").html_safe
+  end
+
   def preload_script(script)
     path = script_asset_path(script)
     preload_script_url(path)