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/editor/src/editor.rs

@@ -4004,6 +4004,10 @@ impl Editor {
             (confirm)(cx);
         }
 
+        if completion.show_new_completions_on_confirm {
+            self.show_completions(&ShowCompletions, cx);
+        }
+
         let provider = self.completion_provider.as_ref()?;
         let apply_edits = provider.apply_additional_edits_for_completion(
             buffer_handle,