Part 4: Implement generic role mapping

This revision 'implements' the 'generic' ARIA role mapping by mapping ARIA 'generic' to Gecko 'section.' This isn't a full implementation of 'generic' - it sidesteps it to avoid dealing with larger 'generic' issues (namely: that there are multiple roles that are 'generic' in Gecko currently). We'll sort that out in a later commit. This revision adds a new web platform test to verify that the generic role actually overrides an element's implicit role.

Differential Revision: https://phabricator.services.mozilla.com/D200133

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1732306 gecko-commit: 55bbf06eb769d45491e170404daa7c55e95c3a0f gecko-reviewers: Jamie