Add support for selectmenu in Form API

Problem/Motivation

An easier to style alternative to select is coming, selectmenu. When it does arrive in supported browsers, we should support it

See https://open-ui.org/components/selectmenu/

Steps to reproduce

Postponed for now, but we could consider https://github.com/luwes/selectmenu-polyfill

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet