Show recently-opened files when autocompleting /file without arguments (#12434)

<img width="1588" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/ea63b046-64d6-419e-8135-4863748b58fa">


Release Notes:

- N/A

crates/assistant_slash_command/src/assistant_slash_command.rs

@@ -25,6 +25,7 @@ pub trait SlashCommand: 'static + Send + Sync {
         &self,
         query: String,
         cancel: Arc<AtomicBool>,
+        workspace: WeakView<Workspace>,
         cx: &mut AppContext,
     ) -> Task<Result<Vec<String>>>;
     fn requires_argument(&self) -> bool;