Add elixir and heex mode

[jsmestad]

I think all that is needed to add the definitions. Let me know if I missed anything.

[renzmann]

Both of these appear to be Emacs major modes, not grammars for elixir/heex. However, if we did have grammar repos listed here, then anyone with those Emacs modes installed would still benefit from treesit-auto

[renzmann]

Both elixir and heex are now included as part of #28