DEV: Don't use js.erb for constants

Adds a new rake task to auto generate a constants.js file with the
constants present. This makes migrating to Ember CLI easier, but also
slightly speeds up asset compilation by having to do less work.

If the constants change you need to run:
`rake javascripts:update_constants`

lib/tasks/javascript.rake

@@ -12,8 +12,24 @@ def library_src
   "#{Rails.root}/node_modules"
 end
 
-task 'javascript:update' do
+task 'javascript:update_constants' => :environment do
+  constants_js = <<~JS
+    // DO NOT EDIT THIS FILE!!!
+    // Update it by running `rake javascript:update_constants`
 
+    export const SearchPriorities = #{Searchable::PRIORITIES.to_json};
+
+    export const SearchPhraseRegexp = '#{Search::PHRASE_MATCH_REGEXP_PATTERN}';
+  JS
+
+  output_path = "#{Rails.root}/app/assets/javascripts/discourse/app/lib/constants.js"
+  File.write(output_path, constants_js)
+  puts "contants.js created"
+  %x{yarn run prettier --write #{output_path}}
+  puts "constants.js prettified"
+end
+
+task 'javascript:update' do
   require 'uglifier'
 
   yarn = system("yarn install")