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_slash_command/src/assistant_slash_command.rs

@@ -25,7 +25,7 @@ pub trait SlashCommand: 'static + Send + Sync {
         &self,
         query: String,
         cancel: Arc<AtomicBool>,
-        workspace: WeakView<Workspace>,
+        workspace: Option<WeakView<Workspace>>,
         cx: &mut AppContext,
     ) -> Task<Result<Vec<String>>>;
     fn requires_argument(&self) -> bool;