vm: allow global proxy reuse across contexts

[koal44]

Cross-document navigation in a browsing context creates a new Window and realm while preserving the identity of its WindowProxy/globalThis. Modeling this behavior with node:vm is currently not possible, as discussed in #855 and #31807.

This PR adds a reuseGlobalProxy option to vm.createContext(). When supplied a global proxy produced with vm.constants.DONT_CONTEXTIFY, Node.js detaches it from the old context and attaches it to a new one.

The implementation was inspired by Chromium’s approach: detaching the old context, then passing the preserved global proxy into the replacement context.

Conceptually:

old_context->DetachGlobal();
new_context = Context::New(/* ... */, old_global_proxy);

[koal44]

I’m closing this version while I continue exploring what an HTML embedder needs from Node's vm API. Using this in my own project showed that global-proxy reuse needs a clearer context lifecycle. I now favor something more like:

const first = vm.createContextHandle();
const windowProxy = first.globalProxy;

first.detachGlobal();

const second = vm.createContextHandle({
  reuseGlobalProxyFrom: first,
});

console.log(second.globalProxy === windowProxy); // true

I'll return once I have a better WPT-backed story for the API.