Nested control: NEW plugin to nest field within checkbox/radio

[GormFrank]

What does this pull request (PR) do?

Provide ability to have a nested field within a checkbox or radio form control.

Additional information

General checklist / Liste de contrôle générale

• Translate documentation to French
• Validate changes against WCAG for accessibility and test for screen readers
• Test against other checkbox and radio button list patterns (may collide with GC-Chckbxrdio in GCweb)
• Define limitations

[GormFrank]

Up here for testing purposes: https://universallabs.org/wet/labs/pr-9316/demos/nested-control/nested-control-en.html

[duboisp]

In the working example, the first bullet items are right aligned instead of left aligned.

[EricDunsworth]

FWIW it looks like it's only happening in Firefox. I checked the Universal labs demo in Edge Chromium earlier and it looked ok in that browser.

Seems similar to #9147 but with a different root cause.

[EricDunsworth]

If this PR ultimately leads to anything viable (i.e. a stable feature - not a provisional "use at your own risk" class or plugin :P), it might be worth modifying the OP or commit message to make it close #8836.

IMO this PR's code is much simpler and more elegant than what @fsnoddy and I proposed in #8836. But it lacks aria-* attributes (some of which would've been brought in by the toggle plugin if #8836 had been finalized).

More research and testing will probably be needed to figure out how best to leverage aria-* attributes in the scenario this PR addresses. For instance, could you get away with just aria-live? Would aria-controls be a better bet, or would screen readers not care about in practice?