This PR updates the Erlang extension with support for installing the
[Erlang Language
Platform](https://github.com/WhatsApp/erlang-language-platform) (`elp`)
language server from the GitHub Release assets.
Release Notes:
- N/A
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.
