Allow completing slash command arguments from extensions (#13240)

This PR extends the extension API with support for completing slash
command arguments for slash commands defined in extensions.

Release Notes:

- N/A

crates/assistant_slash_command/src/assistant_slash_command.rs

@@ -23,7 +23,7 @@ pub trait SlashCommand: 'static + Send + Sync {
     fn description(&self) -> String;
     fn menu_text(&self) -> String;
     fn complete_argument(
-        &self,
+        self: Arc<Self>,
         query: String,
         cancel: Arc<AtomicBool>,
         workspace: Option<WeakView<Workspace>>,