This makes the build system consistent with Godot again. Also fix CMake build to define REAL_T_IS_DOUBLE when precision=double.
Changes the `<godot/gdextension_interface.h>` include to simply `<gdextension_interface.h>`. Refactor and better document the SCons and CMake logic around setting the paths to the header and API JSON file.
Adds a CI static check for it.
Thaddeus Crews
Ricardo Buring
Rémi Verschelde
Fabio Alessandrelli