DEV: Upgrading Discourse to Zeitwerk (#8098)

Zeitwerk simplifies working with dependencies in dev and makes it easier reloading class chains. 

We no longer need to use Rails "require_dependency" anywhere and instead can just use standard 
Ruby patterns to require files.

This is a far reaching change and we expect some followups here.

spec/lib/theme_javascript_compiler_spec.rb

@@ -2,8 +2,6 @@
 
 require 'rails_helper'
 
-require_dependency 'theme_javascript_compiler'
-
 describe ThemeJavascriptCompiler do
 
   let(:theme_id) { 22 }