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.
Non-exhaustive list of case-sensitive renames:
GDExtension -> GDNative
GDNATIVE -> GDEXTENSION
gdextension -> gdnative
ExtensionExtension -> Extension (for where there was GDNativeExtension)
EXTENSION_EXTENSION -> EXTENSION (for where there was GDNATIVE_EXTENSION)
gdnlib -> gdextension
gdn_interface -> gde_interface
gdni -> gde_interface
Chris Cranford
Rémi Verschelde
Gilles Roudière
Emmanuel Leblond
Aaron Franke
bruvzg