Support for transpiling .js files (#9160)

* Remove some `.es6` from comments where it does not matter

* Use a post processor for transpilation

This will allow us to eventually use the directory structure to
transpile rather than the extension.

* FIX: Some errors and clean up in confirm-new-email

It would throw an error if the webauthn element wasn't present.
Also I changed things so that no-module is not explicitly
referenced.

* Remove `no-module`

Instead we allow a magic comment: `// discourse-skip-module` to prevent
the asset pipeline from creating a module.

* DEV: Enable babel transpilation based on directory

If it's in `app/assets/javascripts/dicourse` it will be transpiled
even without the `.es6` extension.

* REFACTOR: Remove Tilt/ES6ModuleTranspiler

plugins/discourse-local-dates/assets/javascripts/discourse-local-dates.js.es6

@@ -1,3 +1,4 @@
+// discourse-skip-module
 (function($) {
   const DATE_TEMPLATE = `
     <span>

plugins/discourse-local-dates/plugin.rb

@@ -6,7 +6,7 @@
 # author: Joffrey Jaffeux
 hide_plugin if self.respond_to?(:hide_plugin)
 
-register_asset 'javascripts/discourse-local-dates.js.no-module.es6'
+register_asset 'javascripts/discourse-local-dates.js.es6'
 register_asset 'stylesheets/common/discourse-local-dates.scss'
 register_asset 'moment.js', :vendored_core_pretty_text
 register_asset 'moment-timezone.js', :vendored_core_pretty_text