This is taken from the abandoned error stacks proposal, which
already serves as the source of truth for the setter. It only requires
the this value to be an object - if it's not an Error object, the getter
returns undefined.
I have not compared this behavior to the non-standard implementations of
the stack property in other engines, but presumably the spec authors
already did that work.
This change gets the Sentry browser SDK working to a point where it can
actually send uncaught exceptions via the API :^)