Add .html.heex extension to HTML+EEX language

[nallwhy]

Description

This PR adds .html.heex extension to HTML+EEX language.

Phoenix LiveView changed its main template extension from .html.leex to .html.heex.
(https://hexdocs.pm/phoenix_live_view/assigns-eex.html)

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%3Aheex&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.heex
    • Sample license(s): MIT
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[nallwhy]

This PR is .html.heex version of #5040.