Extract SlashCommand trait from assistant (#12252)

This PR extracts the `SlashCommand` trait (along with the
`SlashCommandRegistry`) from the `assistant` crate.

This will allow us to register slash commands from extensions without
having to make `extension` depend on `assistant`.

Release Notes:

- N/A

crates/assistant/Cargo.toml

@@ -12,6 +12,7 @@ doctest = false
 [dependencies]
 anyhow.workspace = true
 anthropic = { workspace = true, features = ["schemars"] }
+assistant_slash_command.workspace = true
 cargo_toml.workspace = true
 chrono.workspace = true
 client.workspace = true