fix(radio-group): value handling

[olaf-k]

This short PR attempts to fix the radio-group's value setting.

In the current code, inferring a radio-group's initial value from a checked radio child does not work properly:

• the form's value is not set
• the radio-group is not focusable (because all its radios get -1 tabindex)

Both of these behaviors stem from the fact that the value setter's logic is not called. This PR corrects that by triggering it in the slot change handler.

I've also taken the opportunity to:

• remove Updates.enqueue from this handler as it defers the component initialization (first call to radiosChanged) to the next event loop tick, which is not great UX if authors want to get the value early (e.g., in a whenDefined call).
• replace direct calls to checkRadio() by radio.click() in radio-group so that keyboard navigation would also emit the proper events.

(Not related but, for some reason, there is code that unchecks a radio when it becomes disabled. I'm not sure that's wanted as it is a strange departure from native behavior, so I also removed that.)

[chrisdholt]

Adding @radium-v to review, thanks @olaf-k!

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