feat: isolate groups

[piscisaureus]

Currently requires the following gn flags to work:

v8_enable_external_code_space = true
v8_enable_pointer_compression = true
v8_enable_pointer_compression_shared_cage = false
v8_enable_sandbox = false

[piscisaureus]

It's unfortunate that IsolateGroup currently does not implement Clone.
This is because the C++ class explicitly deletes the copy constructor.

We could probably work around this somehow, although a fairly naive hack that calls internal_isolate_group->Acquire() didn't work when I tried.

[devsnek]

wow this looks identical to my local changes to this lol. could you update the module_snapshot test to use a new group? that will prevent it from being flakey when run without nextest.

[piscisaureus]

wow this looks identical to my local changes to this lol. could you update the module_snapshot test to use a new group? that will prevent it from being flakey when run without nextest.

It doesn't currently build with multiple group support in CI (requires local .gn changes).

[piscisaureus]

Unfortunately, it doesn't seem possible to use isolate groups with pointer compression disabled.
If I neuter the CanCreateNewGroups check in V8, it fails as follows:

#
# Fatal error in ../../../v8/src/objects/js-objects.cc, line 798
# Debug check failed: properties == roots.empty_fixed_array() || properties == roots.empty_property_dictionary() || properties == roots.empty_swiss_property_dictionary().
#
#
#
#FailureMessage Object: 0x16e782698

[joshbetz]

Is there still interest in bringing IsolateGroup support to rusty_v8? If so, I can try to refresh this PR.