[dropdown] Add Dropdown and OptionsMenu components
- Add OptionsMenu, which displays a list of options and implements keyboard navigation
- Add OptionsMenuItem type to describe items
- Add Dropdown, which displays the selected item and expands an OptionsMenu on click. Two named slots are provided to customize rendering.
Both OptionsMenu and Dropdown use v-model to communicate the ID of the
selected option to the parent component.
Bug: T280712
Change-Id: I3463c8563d587f6adf691d67de786cdf43c96675