Add elixir as one of the dev options via mise

[David Heinemeier Hansson]

I think there's an adsf plugin that can be used to make this happen. Just need someone who actually knows elixir to test it out that everything works as it should.

[Jeffrey Guenther]

mise use --global erlang@latest
mise use --global elixir@latest

To test, you can run iex in your shell after install.