This PR adds `language_servers` settings for Elixir and HEEX to ensure
they both only use Elixir LS by default.

Eventually we'd like to have these provided by the Elixir extension, but
there are some outstanding questions on the design.

For now we can just use the built-in default settings to ensure a good
out-of-the-box experience for Elixir users.

Release Notes:

- N/A