Autocomplete commands that don't require access to workspace in prompt library (#12674)

This is useful to autocomplete prompts when writing a new one in the
prompt library.

Release Notes:

- N/A

crates/assistant/src/slash_command/prompt_command.rs

@@ -31,7 +31,7 @@ impl SlashCommand for PromptSlashCommand {
         &self,
         query: String,
         _cancellation_flag: Arc<AtomicBool>,
-        _workspace: WeakView<Workspace>,
+        _workspace: Option<WeakView<Workspace>>,
         cx: &mut AppContext,
     ) -> Task<Result<Vec<String>>> {
         let store = PromptStore::global(cx);