Make AbstractAliasSpecifier public

[JamesWrigley]

I think this should have already been the case since it's a documented type.

Would it be possible to make a new release with it?

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API