Add setting to allow disabling the Assistant (#9706)

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.

crates/zed/src/zed.rs

@@ -3073,6 +3073,7 @@ mod tests {
             notifications::init(app_state.client.clone(), app_state.user_store.clone(), cx);
             workspace::init(app_state.clone(), cx);
             Project::init_settings(cx);
+            command_palette::init(cx);
             language::init(cx);
             editor::init(cx);
             project_panel::init_settings(cx);