Add a slash command for automatically retrieving relevant context (#17972)

* [x] put this slash command behind a feature flag until we release
embedding access to the general population
* [x] choose a name for this slash command and name the rust module to
match

Release Notes:

- N/A

---------

Co-authored-by: Jason <jason@zed.dev>
Co-authored-by: Richard <richard@zed.dev>
Co-authored-by: Jason Mancuso <7891333+jvmncs@users.noreply.github.com>
Co-authored-by: Richard Feldman <oss@rtfeldman.com>

crates/evals/src/eval.rs

@@ -438,7 +438,7 @@ async fn run_eval_project(
             loop {
                 match cx.update(|cx| {
                     let project_index = project_index.read(cx);
-                    project_index.search(query.query.clone(), SEARCH_RESULT_LIMIT, cx)
+                    project_index.search(vec![query.query.clone()], SEARCH_RESULT_LIMIT, cx)
                 }) {
                     Ok(task) => match task.await {
                         Ok(answer) => {