This CL refactors WasmContinuationObject to have a direct
ExternalPointer to the jmpbuf structure instead of using a Foreign.
This in turn makes it possible to use a unique pointer tag for that
external pointer when the sandbox is enabled.

Bug: v8:10391, v8:12949
Change-Id: I25528bd8aaffb32dd617440d3ccb77d319894a38
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3805061Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82238}