[AST] NFC: Attr.def clean up (#16031)

1) Formalize "OnAccessor". A hack used to alias this to "OnFunc".
2) New aggregates: OnNominalType, OnGenericType, OnAbstractFunction.
3) Consistent and self-documented (albeit custom) style to ease code review/audits.
4) Removes a few cases of `OnAccessor` based on pull request #16031 feedback.

Author: davezarzycki