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.toml

@@ -430,7 +430,7 @@ zlog_settings = { path = "crates/zlog_settings" }
 # External crates
 #
 
-agent-client-protocol = { version = "0.2.0-alpha.3", features = ["unstable"] }
+agent-client-protocol = { version = "0.2.0-alpha.4", features = ["unstable"]}
 aho-corasick = "1.1"
 alacritty_terminal = { git = "https://github.com/zed-industries/alacritty.git", branch = "add-hush-login-flag" }
 any_vec = "0.14"