Report when aria-attr is case sensitive and user uses mixed/upper case

https://github.com/dequelabs/axe-core/pull/5224 added case sensitive information to all `boolean` and `mntokens` attributes. We should report needs review any time an attribute is case sensitive and the value is not all lowercase. I think it might make the most sense as a new check to `aria-allowed-attr` similar to the check `aria-unsupported-attr`.

Start by reviewing PR 5224 to understand how case-sensitive information is represented for boolean and mntokens attributes. Then inspect the aria-allowed-attr check alongside aria-unsupported-attr and its tests. Done means mixed- or uppercase values for case-sensitive attributes are reported as needing review, while lowercase values are not.

Written by the indexing model from the issue text.