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/agent/src/context_picker/completion_provider.rs

@@ -874,7 +874,7 @@ mod tests {
         let editor = workspace.update_in(&mut cx, |workspace, window, cx| {
             let editor = cx.new(|cx| {
                 Editor::new(
-                    editor::EditorMode::Full,
+                    editor::EditorMode::full(),
                     multi_buffer::MultiBuffer::build_simple("", cx),
                     None,
                     window,