zed_extension_api: Add default implementation for language_server_command (#13234)

This PR adds a default implementation for the `language_server_command`
method on the `Extension` trait.

This will allow for extensions to be defined without having to implement
this method, which will be useful for extensions that may just want to
provide slash commands.

Release Notes:

- N/A

crates/extension_api/src/extension_api.rs

@@ -66,9 +66,11 @@ pub trait Extension: Send + Sync {
     /// language.
     fn language_server_command(
         &mut self,
-        language_server_id: &LanguageServerId,
-        worktree: &Worktree,
-    ) -> Result<Command>;
+        _language_server_id: &LanguageServerId,
+        _worktree: &Worktree,
+    ) -> Result<Command> {
+        Err("`language_server_command` not implemented".to_string())
+    }
 
     /// Returns the initialization options to pass to the specified language server.
     fn language_server_initialization_options(