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/extension/src/extension_slash_command.rs

@@ -39,6 +39,7 @@ impl SlashCommand for ExtensionSlashCommand {
         &self,
         _query: String,
         _cancel: Arc<AtomicBool>,
+        _workspace: WeakView<Workspace>,
         _cx: &mut AppContext,
     ) -> Task<Result<Vec<String>>> {
         Task::ready(Ok(Vec::new()))