Investigate pre-parsing `aria-*` attributes according to their value type.

[Jym77]

According to Mapping WAI-ARIA Value types to languages, aria-* attribute of type true/false, true/false/undefined and tristate should be parsed as enumerated attributes, hence case-insensitively, which seems to not be the case currently 😕

We've quick-fixed R54 in #1083 but a generic solution might still be needed.

Other attributes should be parsed according to their value type so that a clean API is offered to access them.

[Jym77]

Find list of affected rules.

R54.