Use-after-free when closing editor while using GDExtension on Windows

[mihe]

Tested versions

Reproducible in: 4.3.rc [739019e]

System information

Windows 11 (10.0.22631)

Issue description

I haven't bisected this or anything, but given the affected code (WindowsUtils::remove_temp_pdbs) I would assume this is a regression caused by #87117.

Similar to #95306, I only noticed this when running the editor under Application Verifier on Windows, which similar to Valgrind lets you know if a use-after-free has occurred, among other things. I haven't noticed any crash when running normally, but I suppose that could maybe happen, given the somewhat random nature of a use-after-free.

Hopefully the problem is apparent from this flow when closing the editor of a project (on Windows specifically) that utilizes a GDExtension:

1. Main::cleanup is called
2. ⚠️ Main::cleanup deletes packed_data (which is the singleton for PackedData) but the singleton isn't zeroed out
3. Main::cleanup calls unregister_core_types
4. unregister_core_types deletes gdextension_manager
5. That unrefs all the GDExtension, thus deleting them
6. GDExtension::~GDExtension calls GDExtension::close_library
7. GDExtension::close_library calls OS_Windows::close_dynamic_library
8. OS_Windows::close_dynamic_library calls OS_Windows::_remove_temp_library
9. OS_Windows::_remove_temp_library calls WindowsUtils::remove_temp_pdbs
10. WindowsUtils::remove_temp_pdbs calls FileAccess::exists
11. ⚠️ FileAccess::exists accesses PackedData::singleton

Steps to reproduce

N/A

Minimal reproduction project (MRP)

I'd be happy to provide an MRP, but I'm fairly sure this should be reproducible in pretty much any godot-cpp project.

[mihe]

I put up a fix in #95311.

CC @dsnopek