editor: Refactor EditorMode::Full (#28546)

This PR lightly refactors the `EditorMode::Full` exposing two new
methods: `is_full` and `set_mode`.

Motivation is to expose fields that modify the behavior when the editor
is in `Full` mode. By using is `mode.is_full()` instead of
`EditorMode::Full` we can introduce new fields without breaking other
places in the code.

Release Notes:

- N/A

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>

crates/vim/src/test.rs

@@ -1729,7 +1729,7 @@ async fn test_folded_multibuffer_excerpts(cx: &mut gpui::TestAppContext) {
             ],
             cx,
         );
-        let mut editor = Editor::new(EditorMode::Full, multi_buffer.clone(), None, window, cx);
+        let mut editor = Editor::new(EditorMode::full(), multi_buffer.clone(), None, window, cx);
 
         let buffer_ids = multi_buffer.read(cx).excerpt_buffer_ids();
         // fold all but the second buffer, so that we test navigating between two