There is a bunch of code that is shared with Godot that we regularly synchronize by copying the Godot version and modifying to work in godot-cpp.

We haven't done that in a while!

The goal of this PR is to completely synchronize most of the shared variant code, ideally, in advance of the Godot 4.4 release. (Although, there is a lot of stuff to work through!)

The process is basically:

1. Delete everything between namespace godot { ... } in godot-cpp's .hpp or .cpp file
2. Copy the corresponding code from Godot
3. Make simple, superficial changes in order to compile. For example, in godot-cpp, we don't use Vector<Vector3> but instead PackedVector3Array, and the EulerOrder enum has different values in godot-cpp, etc. No large-scale changes were made to the Godot code!

I have omitted a couple of things:

• The HSL methods on Color: they depend on a third-party library (ok_color.h) which we'll need to figure out how to include with godot-cpp
• TypedDictionary and TypedArray: these will need special care
• char_string.hpp: This file differs from Godot's by using more templates, leading to less code duplication. There is PR Core: Integrate CharStringT godot#98439 to port these changes into Godot, so rather than revert to the Godot version, we'll wait for Godot to update to this version :-)

Marking as a DRAFT because I haven't gotten through everything yet

UPDATE: I've decided to limit this to the variant code (not all shared code) and to omit a couple of things, in order to reduce the scope of this PR, and get it out of DRAFT sooner.

Fixes #1644