Customizable face for numbers

[jsmestad]

Background

I had made a patch to doom-emacs (see hlissner/doom-emacs#3613) to highlight 10_000 correctly after the Elixir formatter had run. As a next step, I wanted to have this functionality built into emacs-elixir mode for those who want to customize how numbers show up.

While I was in there, I did some general cleanup based on best practices and the Emacs Lisp docs.

Changes

• Expose all variables as customizable
• Increment version to 2.4.0
• Expose font faces under the elixir group rather than font-lock-faces
• Add elixir-numbers-face
  • Set to default so it does not conflict with today's functionality
• Move elixir-format variables under the same elixir group.
• Add lexical-binding: t to top of the file (from style guide)

Discussion

Is default appropriate?

[jsmestad]

I changed up the test matrix as it was failing on Emacs 24.5 (same with master).

Looking around at other major modes, most have moved up to a minimum support of Emacs 26.3, so I did the same. I wanted to test Emacs 27.1 and Emacs 28.0.50, but EVM lacks support for these releases at the moment.

[jsmestad]

@Trevoke and @victorolinasc I rolled back all the attempted CI fixes. Please take a look when you get the time 👍