Enhancements to includes in core data structures

[aaronfranke]

These changes fix things I noticed while working on syncing up the engine's data structures and godot-cpp.

• Prefer forward-declaring String and Vector instead of including them where possible.
• Only include what's needed (ex: Vector4 does not need Vector3, it was a blind copy/paste from Quaternion).
  • Due to this, Vector is no longer implicitly included in Transform3D, so add an explicit one.
• Add missing _NO_DISCARD_ to Projection.
• Add some explicit (real_t) casts (for consistency with godot-cpp's Math::min requirements).
• Some spacing and comment tweaks.

[akien-mga]

Thanks!