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);