Introduce /search command to assistant (#12372)

This pull request introduces semantic search to the assistant using a
slash command:


https://github.com/zed-industries/zed/assets/482957/62f39eae-d7d5-46bf-a356-dd081ff88312

Moreover, this also adds a status to pending slash commands, so that we
can show when a query is running or whether it failed:

<img width="1588" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/e8d85960-6275-4552-a068-85efb74cfde1">

I think this could be better design-wise, but seems like a pretty good
start.

Release Notes:

- N/A

crates/zed/src/zed.rs

@@ -1017,6 +1017,7 @@ mod tests {
         let workspace_1 = cx
             .read(|cx| cx.windows()[0].downcast::<Workspace>())
             .unwrap();
+        cx.run_until_parked();
         workspace_1
             .update(cx, |workspace, cx| {
                 assert_eq!(workspace.worktrees(cx).count(), 2);