Switching stack affects all instances, therefore make the active
continuation a root object instead of an instance field.

Bug: v8:12191
Change-Id: Ia521bf4e7c3cbca035f916e47804c2f75bbd8f18
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3268296Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77876}