Elixir: inject Markdown into docs, remove h sigil HEEx injection

[connorlay]

@the-mikedavis I figured I'd checkout Helix and see how it compares to Neovim. Consider me a convert 😆

This PR contains two changes:

• injects Markdown, the official documentation format, into Elixir
• Removes the non-existent h sigil from Elixir language injections. Phoenix Live View only supports the uppercase H sigil for HEEx.

Please let me know if there are any tests or documentation that need to be updated. Thanks!

[the-mikedavis]

Ah TIL HEEx is limited to ~H, that makes sense 😄

Glad we could steal you from nvim - welcome! 🥳

[the-mikedavis]

W.r.t. tests, I think it might be good in the long-run to have highlight tests in the CI and/or cargo testsuite. For now I think it would be a little heavy since we're adding new languages very often and tree-sitter queries are already a lot to learn if you've never seen tree-sitter before

[archseer]

Consider me a convert

I see our Helix evangelism strikeforce is just as strong as the Rust one :D