Editor toolbar configuration (#7338)

Adds settings for hiding breadcrumbs and quick action bar from
the editor toolbar. If both elements are hidden, the toolbar disappears
completely.

Example:

```json
"toolbar": {
  "breadcrumbs": true,
  "quick_actions": false
}
```

- It intentionally doesn't hide breadcrumbs in other views (for
instance, in the search result window) because their usage there differ
from the main editor.
- The editor controls how breadcrumbs are displayed in the toolbar, so
implementation differs a bit for breadcrumbs and quick actions bar.

Release Notes:

- Added support for configuring the editor toolbar ([4756](https://github.com/zed-industries/zed/issues/4756))

crates/workspace/src/toolbar.rs

@@ -127,7 +127,7 @@ impl Render for Toolbar {
                             h_flex()
                                 // We're using `flex_none` here to prevent some flickering that can occur when the
                                 // size of the left items container changes.
-                                .flex_none()
+                                .when_else(has_left_items, Div::flex_none, Div::flex_auto)
                                 .justify_end()
                                 .children(self.right_items().map(|item| item.to_any())),
                         )