assistant: Don't require a worktree to run slash commands (#15655)

This PR fixes an issue where slash commands were not able to run when
Zed did not have any worktrees opened.

This requirement was only necessary for slash commands originating from
extensions, and we can enforce the presence of a worktree just for
those:

<img width="378" alt="Screenshot 2024-08-01 at 5 01 58 PM"
src="https://github.com/user-attachments/assets/38bea947-e33b-4c64-853c-c1f36c63d779">

Release Notes:

- N/A

crates/assistant_slash_command/src/assistant_slash_command.rs

@@ -49,7 +49,7 @@ pub trait SlashCommand: 'static + Send + Sync {
         //
         // It may be that `LspAdapterDelegate` needs a more general name, or
         // perhaps another kind of delegate is needed here.
-        delegate: Arc<dyn LspAdapterDelegate>,
+        delegate: Option<Arc<dyn LspAdapterDelegate>>,
         cx: &mut WindowContext,
     ) -> Task<Result<SlashCommandOutput>>;
 }