This PR adds a new `assistant.enabled` setting that controls whether the
Zed Assistant is enabled.

Some users have requested the ability to disable the AI-related features
in Zed if they don't use them. Changing `assistant.enabled` to `false`
will hide the Assistant icon in the status bar (taking priority over the
`assistant.button` setting) as well as filter out the `assistant:`
actions.

The Assistant is enabled by default.

Release Notes:

- Added an `assistant.enabled` setting to control whether the Assistant
is enabled.