This moves the weak list of instances from {WasmModuleObject} to the
corresponding {Script} object. The list is used solely for breakpoints
which are intended to affect all instances belonging to a given script,
hence the new placement of the list is a preparation to fully support
per-script breakpoints.

R=clemensb@chromium.org
BUG=v8:6847,chromium:893069

Change-Id: I52315e0ba1e5e5021f55bf05d8cb0f01bf9f0fbb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1847359
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64191}