Project: expression-editor
Task (challenging): Add keyboard support for completion navigation and inserting.

The goal of this feature is to enable users to:

- Navigate through this list using the up and down arrow keys.
- Insert the highlighted completion into the editor using the Enter key.

# Details
- When completion popover appears, select first item by default
- Selected completion item should have a different background color from unselected one.
- Loop from the last item back to the first (and vice versa) when navigating beyond the end (or beginning) of the list.