DEV: Merge root JS packages (#25857)

Before this commit, we had a yarn package set up in the root directory and also in `app/assets/javascripts`. That meant two `yarn install` calls and two `node_modules` directories. This commit merges them both into the root location, and updates references to node_modules.

A previous attempt can be found at https://github.com/discourse/discourse/pull/21172. This commit re-uses that script to merge the `yarn.lock` files.

Co-authored-by: Jarek Radosz <jradosz@gmail.com>

spec/lib/discourse_js_processor_spec.rb

@@ -159,11 +159,7 @@ RSpec.describe DiscourseJsProcessor do
 
     let(:mini_racer) do
       ctx = MiniRacer::Context.new
-      ctx.eval(
-        File.open(
-          "#{Rails.root}/app/assets/javascripts/node_modules/handlebars/dist/handlebars.js",
-        ).read,
-      )
+      ctx.eval(File.open("#{Rails.root}/node_modules/handlebars/dist/handlebars.js").read)
       ctx.eval(helpers)
       ctx
     end