chore: stop leaking v8 environment in main process

[nornagon]

Description of Change

This changes our shutdown procedure to include cleaning up the v8 Isolate. The bug referenced in the comment appears to no longer be an issue.

Two things broke when I made this change, both discovered via ASan:

1. Browser was being destroyed after the Isolate, which meant it was attempting to clean up its Global handles, but v8 had already cleaned them up.
2. There were some global variables of type v8::Global, which were crashing on shutdown because their destructors were being called during C++'s finalization stage. I've switched those globals to base::NoDestructor to prevent those destructors from being called. They might be better stored in gin::PerIsolateData or on some other object with a more clearly defined lifetime. However, at present we only ever have a single v8 isolate in the main process, so that problem can be solved another time.

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• PR title follows semantic commit guidelines

Release Notes

Notes: none

[deepak1556]

Nice 💯

[release-clerk]

No Release Notes