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/semantic_index/examples/index.rs

@@ -98,7 +98,7 @@ fn main() {
                 .update(|cx| {
                     let project_index = project_index.read(cx);
                     let query = "converting an anchor to a point";
-                    project_index.search(query.into(), 4, cx)
+                    project_index.search(vec![query.into()], 4, cx)
                 })
                 .unwrap()
                 .await