Add cross-origin opener policy

This commit adds the notion of cross-origin opener policy (COOP). COOP
allows websites to restrict which origins they share their browsing
context group with. annevk wrote a first draft of the behavior of COOP
here: https://gist.github.com/annevk/6f2dd8c79c77123f39797f6bdac43f3e.
This takes that draft and merges it into the spec, with many updates
along the way.

Closes whatwg#3740. Closes whatwg#4580. Closes whatwg#4921. Closes whatwg#5172.

Co-authored-by: clamy <clamy@chromium.org>
Co-authored-by: Anne van Kesteren <annevk@annevk.nl>
Co-authored-by: Domenic Denicola <d@domenic.me>