Specify wheel event groups

[dlrobertson]

Specify the behavior of wheel event groups.

Closes #338

The following tasks have been completed:

• [x] Confirmed there are no ReSpec/BikeShed errors or warnings.
• [x] Modified Web platform tests: https://github.com/web-platform-tests/wpt/pull/37445

Implementation commitment:

• [ ] WebKit: Passes tests in https://github.com/web-platform-tests/wpt/pull/37445
• [ ] Chromium: Passes tests in https://github.com/web-platform-tests/wpt/pull/37445
• [x] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=1168182)

[dlrobertson]

See https://github.com/web-platform-tests/wpt/pull/37445#issuecomment-1609127452, in particular:

I think I'd still prefer to call this thing something else than "event group". It is more like a wheel transaction or some such.

I'll update the text here to use "wheel transaction" instead of "wheel event group"

CC @smaug----

[dlrobertson]

@zcorpan @garykac rebased

[dlrobertson]

\o/ Thanks @zcorpan and @garykac!