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/assistant/Cargo.toml

@@ -41,6 +41,7 @@ regex.workspace = true
 rope.workspace = true
 schemars.workspace = true
 search.workspace = true
+semantic_index.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 settings.workspace = true