R1:ead5f4fd9c03

Add an "Accepting reviewers" Herald field for commits

Summary:
See PHI262. Fixes T12578. Although this is a bit niche and probably better accomplished through advisory/soft measures ("Add blocking reviewers") in most cases, it isn't difficult to implement and doesn't create any technical or product tension.

If installs write a rule that blocks commits, that will probably also naturally lead them to an "add reviewers" rule anyway.

Also, allow packages to be hit with the typeahead. They're valid reviewers…