assistant: Add implementation for `/delta` argument completion (#19693)
Marshall Bowers
created
This PR fixes a panic that could occur when trying to complete arguments
for the `/delta` slash command.
We were using `unimplemented!()` instead of providing a default no-op
implementation like we do for other slash commands that do not support
completing arguments.
Closes https://github.com/zed-industries/zed/issues/19686.
Release Notes:
- Fixed a panic that could occur when trying to complete arguments with
the `/delta` command.