FEATURE: Plugin support for transpiling regular .js files (#9398)

This adds support for a new piece of metadata to your plugin.rb
files. If you add:

```
transpile_js: true
```

Then Discourse will support transpilation of assets in your
`assets/javascripts` directory. Previously they had to be named
`.js.es6` but now regular `.js` will work.

Note this is opt-in because some plugins currently have `.js` files in
app/assets that are not meant to be transpiled.

Going forward all plugins should migrate to this setting as they are
comfortable able to do so.

lib/plugin/metadata.rb

@@ -78,7 +78,7 @@ class Plugin::Metadata
     "discourse-internet-explorer"
   ])
 
-  FIELDS ||= [:name, :about, :version, :authors, :url, :required_version]
+  FIELDS ||= [:name, :about, :version, :authors, :url, :required_version, :transpile_js]
   attr_accessor(*FIELDS)
 
   def self.parse(text)