This gets rid of a couple FIXMEs and allows reusing the logic of
validating this field between different algorithms. While we're here,
expand its logic to match the constraints as outlined in RFC 7517.
The main motivation behind this is to remove JS specifics of the Realm
from the implementation of the Heap.
As a side effect of this change, this is a bit nicer to read than the
previous approach, and in my opinion, also makes it a little more clear
that this method is specific to a JavaScript Realm.
