This PR should fix the issue #1872, where a select with both multiple and required props is locked in a form when more than 1 option is selected.

I created a story for this specific issue, and it should support both controlled and uncontrolled values.

• default value works as expected
• native select is kept for a11y, but uses a v-model so that Vue itself handles the change between string/string array
• 'change' event bubbling to the form
• on submit, the selected options can be get with formData.getAll('name')

Things I'm not sure about :

• options is stored as a Map instead of a Set (I'm not sure that storing objects in a Set ensures that there are unique?)
• Call to rootContext.onValueChange() removed (side-effects?)

Things I need help with:
The <SelectValue> text flickers when opening or closing the select, because the Map temporarily loses all its keys before they’re re-added. Its size goes down to 0 when the content is closing (since ItemText emits an event on unmount), and then all the options are added again.
The issue is that during the frame between removal and addition, the placeholder briefly appears. However, this might be a separate issue, since it can also be observed in the Select component on the Reka UI docs (although it’s very short — blink and you’ll miss it!).

Fix #1872