Add LEEx Support

[oohnoitz]

Description

This adds support for Live EEx templates. It is primarily used with Phoenix LiveView library as a template engine.

The sample included is the one generated by the Phoenix Framework generator's mix phx.gen.live command. The source link does contain some extra % to help escape some characters for the generator to output the content properly.

See https://hexdocs.pm/phoenix_live_view/Phoenix.LiveView.Engine.html#content for documentation of the template engine.

Checklist:

• I am associating a language with a new file extension.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?q=extension%3Aleex+phx+NOT+nothack&type=Code (with some LiveView-specific features)
      • https://github.com/search?q=extension%3Aleex+NOT+nothack&type=Code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/phoenixframework/phoenix/blob/master/priv/templates/phx.gen.live/form_component.html.leex
    • Sample license(s): MIT
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[lildude]

Usage checks out. 👍