This PR extracts Erlang support into an extension and removes the
built-in Erlang support from Zed.
Tested using a Nix shell:
```
nix-shell -p erlang-ls
```
Release Notes:
- Removed built-in support for Erlang, in favor of making it available
as an extension. The Erlang extension will be suggested for download
when you open a `.erl` or `.hrl` file.
