Add @abi proposal (#2768)

* Add `@abi` proposal

* Take corrections from Ben Rimmington

Co-authored-by: Ben Rimmington <me@benrimmington.com>

* Assign SE-0476 and put into active review.

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Co-authored-by: Ben Rimmington <me@benrimmington.com>
Co-authored-by: Holly Borla <hborla@apple.com>

Author: 3 people