[DRAFT] GDExtension: Check conversions to/from GDExtension pointer types

[dsnopek]

This PR builds on top of #107845

It generates the GDExtension interface header differently than the "legacy" version, swapping void * for opaque structs, in an attempt to allow the compiler to validate conversions between Godot types and the types used by the GDExtension interface

So, rather than using C-style casts or reinterpret_cast()s at each conversion, we would use the new to_gdextension() and from_gdextension() functions, which should only be able to perform valid conversions.

While we haven't had a bug from invalid conversions recently, we have had numerous such bugs in the past, and this should make that impossible at compile time. It would also make life somewhat easier for contributors, because it should eliminate the question of which cast to use, or which GDExtension type is correct. Since they are all void * in the legacy header and we're using casts that do no checking, it's really easy to use the wrong one, and we rely on imperfect human review to check it.

In fact, in this PR, you can see a handful of cases where this change turned up the wrong GDExtension type being used!

NOTE: This PR is unfinished which is why it's marked as DRAFT. However, I wanted to share it, in order to show an important thing that PR #107845 would allow us to do