Update checked for radio buttons without group.

[brunnerh]

Also make radio button group name a required property (can change this again, if there is a good reason not to do this).

Unfortunately the change event does only fire for the newly selected radio button, not for the ones that are automatically unselected; this is why I used document.getElementsByName to find the associated ones.

Fixes #1879.

[brunnerh]

Also make radio button group name a required property (can change this again, if there is a good reason not to do this).

The skeleton UIs might be a good reason 🤔.

<RadioButtonGroup legendText="Storage tier (disk)" name="skeleton">
  <RadioButtonSkeleton />
  <RadioButtonSkeleton />
  <RadioButtonSkeleton />
</RadioButtonGroup>

[metonym]

@brunnerh Happy to revisit this. In the context of implementing #2225, what does this PR do?

[brunnerh]

It synchronizes the checked state across members of the implicit radio button group.
See comments on your recent PR.

[metonym]

Why does it need to use the DOM to do this? Why can't RadioButtonGroup handle this?

[brunnerh]

Because there is no RadioButtonGroup, this is specifically about usage without that component, e.g. because the radio buttons are far apart with other UI in-between. RadioButtonGroup does not just provide context, it also does layout (which arguably maybe it should not do in the first place).