Extract Erlang support into an extension (#9974)

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.

extensions/erlang/src/erlang.rs

@@ -0,0 +1,27 @@
+use zed_extension_api::{self as zed, Result};
+
+struct ErlangExtension;
+
+impl zed::Extension for ErlangExtension {
+    fn new() -> Self {
+        Self
+    }
+
+    fn language_server_command(
+        &mut self,
+        _config: zed::LanguageServerConfig,
+        worktree: &zed::Worktree,
+    ) -> Result<zed::Command> {
+        let path = worktree
+            .which("erlang_ls")
+            .ok_or_else(|| "erlang_ls must be installed and available on your $PATH".to_string())?;
+
+        Ok(zed::Command {
+            command: path,
+            args: Vec::new(),
+            env: Default::default(),
+        })
+    }
+}
+
+zed::register_extension!(ErlangExtension);