Embedded Ruby support - redux

Includes all the base commits from PR #4320 and addresses the dangling code review comments there, plus a few other tweaks:

* Maps .js.erb to plain JS and .scss.erb to plain SCSS
* Tweak the file extension list for ERB HTML mode -- from what I've seen [and read](https://www.ruby-forum.com/topic/120053), .erb.html isn't really used while the reverse is common.
* Adds block comment support for ERB HTML
* Don't use a generic "erb" language ID, since there are now several outer-language-specific ERB mappings