Implement built-in classes Vector4, Vector4i and Projection.
* Two versions of Vector4 (float and integer).
* A Projection class, which is a 4x4 matrix specialized in projection types.
These types have been requested for a long time, but given they were very corner case they were not added before.
Because in Godot 4, reimplementing parts of the rendering engine is now possible, access to these types (heavily used by the rendering code) becomes a necessity.
**Q**: Why Projection and not Matrix4?
**A**: Godot does not use Matrix2, Matrix3, Matrix4x3, etc. naming convention because, within the engine, these types always have a *purpose*. As such, Godot names them: Transform2D, Transform3D or Basis. In this case, this 4x4 matrix is _always_ used as a _Projection_, hence the naming.
* Map is unnecessary and inefficient in almost every case.
* Replaced by the new HashMap.
* Renamed Map to RBMap and Set to RBSet for cases that still make sense
(order matters) but use is discouraged.
There were very few cases where replacing by HashMap was undesired because
keeping the key order was intended.
I tried to keep those (as RBMap) as much as possible, but might have missed
some. Review appreciated!
* Changed to use the same stages as extensions.
* Makes the initialization more coherent, helping solve problems due to lack of stages.
* Makes it easier to port between module and extension.
* removed the DRIVER initialization level (no longer needed).
Added the ability to import scenes as AnimationLibrary
* Completes implementation of https://github.com/godotengine/godot-proposals/issues/4296
* Helps if you want to export animations to a separate file (say a GLTF) to avoid re-importing/exporting them every time the model changes.
* Helps if you simply want to have animations using a dummy model, which can be shared across multiple models.
Creates a secondary scene importer used only for animations.
**NOTE**: A new flag for scene importer: EditorSceneFormatImporter.IMPORT_DISCARD_MESHES_AND_MATERIALS has been added, to hint importers that they should skip meshes and animations (and hence make importing faster). It is not implemented in any importer yet, this should be done in a separate PR.
* Instead of containing single animations, AnimationPlayer now contains libraries.
* Libraries, in turn, contain the animations.
This paves the way for implementing the possibility of importing scenes as animation libraries, finally allowing to import animations separate from the 3D models.
Missing (will be done on separate PRs):
* Make it possible to import scenes (dae/fbx/gltf) as animation libraries.
* Make it possible for AnimationTree to import animation libraries on its own, so it does not rely on AnimationPlayer for everything.
