[Feature request] Dropdown accessibility improvements

[nvms]

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When a dropdown is opened:

• Tab should be disabled until the dropdown is closed.
• Escape should close the dropdown.
• Array keys should navigate through dropdown items, assuming they are list items.
• On mobile, maybe this should open as a scrollable element, docked to the bottom of the viewport. A drawer?

When a dropdown is closed:

• It should retain its focused state.