src: keep main-thread Isolate attached to platform during Dispose

This works around a situation in which the V8 WASM code calls
into the platform while the Isolate is being disposed.

This goes against the V8 API constract for `v8::Platform`.
In lieu of a proper fix, it should be okay to keep the Isolate
registered; the race condition fixed by 25447d82 cannot
occur for the `NodeMainInstance`’s Isolate, as it is the last
one to exit in any given Node.js process.

This partially reverts 25447d82.

Refs: https://github.com/nodejs/node/pull/30909
Refs: https://github.com/nodejs/node/issues/31752

PR-URL: https://github.com/nodejs/node/pull/31795


Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>