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>

lib/ember_cli.rb

@@ -41,8 +41,7 @@ class EmberCli < ActiveSupport::CurrentAttributes
   def self.ember_version
     @version ||=
       begin
-        ember_source_package_raw =
-          File.read("#{Rails.root}/app/assets/javascripts/node_modules/ember-source/package.json")
+        ember_source_package_raw = File.read("#{Rails.root}/node_modules/ember-source/package.json")
         JSON.parse(ember_source_package_raw)["version"]
       end
   end

lib/highlight_js.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module HighlightJs
-  HIGHLIGHTJS_DIR ||= "#{Rails.root}/app/assets/javascripts/node_modules/@highlightjs/cdn-assets/"
+  HIGHLIGHTJS_DIR ||= "#{Rails.root}/node_modules/@highlightjs/cdn-assets/"
   VERSION ||= 1 # bump to invalidate caches following core changes
 
   def self.languages

lib/js_locale_helper.rb

@@ -332,9 +332,7 @@ module JsLocaleHelper
         @ctx ||=
           begin
             ctx = MiniRacer::Context.new(timeout: 15_000, ensure_gc_after_idle: 2000)
-            ctx.load(
-              "#{Rails.root}/app/assets/javascripts/node_modules/messageformat/messageformat.js",
-            )
+            ctx.load("#{Rails.root}/node_modules/messageformat/messageformat.js")
             ctx
           end
       )

lib/pretty_text.rb

@@ -66,10 +66,12 @@ module PrettyText
     end
 
     root_path = "#{Rails.root}/app/assets/javascripts"
-    ctx.load("#{root_path}/node_modules/loader.js/dist/loader/loader.js")
-    ctx.load("#{root_path}/node_modules/markdown-it/dist/markdown-it.js")
+    node_modules = "#{Rails.root}/node_modules"
+    md_node_modules = "#{Rails.root}/app/assets/javascripts/discourse-markdown-it/node_modules"
+    ctx.load("#{node_modules}/loader.js/dist/loader/loader.js")
+    ctx.load("#{md_node_modules}/markdown-it/dist/markdown-it.js")
     ctx.load("#{root_path}/handlebars-shim.js")
-    ctx.load("#{root_path}/node_modules/xss/dist/xss.js")
+    ctx.load("#{node_modules}/xss/dist/xss.js")
     ctx.load("#{Rails.root}/lib/pretty_text/vendor-shims.js")
 
     ctx_load_directory(