acp: Add support for slash commands (#37304)

Depends on
https://github.com/zed-industries/agent-client-protocol/pull/45

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Agus Zubiaga <agus@zed.dev>

Cargo.lock

@@ -195,9 +195,9 @@ dependencies = [
 
 [[package]]
 name = "agent-client-protocol"
-version = "0.2.0-alpha.3"
+version = "0.2.0-alpha.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4ec42b8b612665799c7667890df4b5f5cb441b18a68619fd770f1e054480ee3f"
+checksum = "603941db1d130ee275840c465b73a2312727d4acef97449550ccf033de71301f"
 dependencies = [
  "anyhow",
  "async-broadcast",