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
Marshall Bowers
254ce74036