Extract ExtensionSlashCommand to assistant_slash_command crate (#20617)

This PR extracts the `ExtensionSlashCommand` implementation to the
`assistant_slash_command` crate.

The slash command related methods have been added to the `Extension`
trait. We also create separate data types for the slash command data
within the `extension` crate so that we can talk about them without
depending on the `extension_host` or `assistant_slash_command`.

Release Notes:

- N/A

crates/assistant_slash_command/Cargo.toml

@@ -13,8 +13,10 @@ path = "src/assistant_slash_command.rs"
 
 [dependencies]
 anyhow.workspace = true
+async-trait.workspace = true
 collections.workspace = true
 derive_more.workspace = true
+extension.workspace = true
 futures.workspace = true
 gpui.workspace = true
 language.workspace = true
@@ -22,6 +24,7 @@ language_model.workspace = true
 parking_lot.workspace = true
 serde.workspace = true
 serde_json.workspace = true
+ui.workspace = true
 workspace.workspace = true
 
 [dev-dependencies]