Add common function to access standards information

Most of our standards stuff is not available through `axe.commons`. We have a few helper functions on `axe.commons.standards`, but there's not a lot for querying the current standards object (`ariaAttrs`, `ariaRoles`, etc.). We should add a few helper functions to return those so you don't have to go through `axe._audit.standards`.

Start by tracing the existing helpers on axe.commons.standards and the current standards object reached through axe._audit.standards, including ariaAttrs and ariaRoles. Identify the appropriate public entry point and existing tests, then add helpers for the requested standards information and verify that callers no longer need the private path.

Written by the indexing model from the issue text.