Extract slash commands to their own crate (#23261)

This PR extracts the slash command definitions out of the `assistant`
crate and into their own `assistant_slash_commands` crate.

Release Notes:

- N/A
8030c0025aMarshall Bowers committed on 1/16/2025, 10:17:07 PM· committed by GitHubparent1a8303b
52 files changedLine totals unavailable