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/extensions_ui/src/extensions_ui.rs

@@ -1,7 +1,6 @@
 mod components;
 mod extension_context_server;
 mod extension_registration_hooks;
-mod extension_slash_command;
 mod extension_suggest;
 mod extension_version_selector;