Remove rpg, ui, editor and scene subfolders; split scripts by platform

Delete src/dusk/rpg, src/dusk/ui, the animation/keyframe modules, the
scene subfolders, the chunk asset loader and ./editor, and strip the
dependent save slot, scene type, engine and test code so the main
binary and tests build again. Add scene/node.c/.h skeleton and move
build scripts into scripts/<platform>/.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

test/CMakeLists.txt

@@ -10,8 +10,6 @@ add_subdirectory(console)
 add_subdirectory(error)
 add_subdirectory(thread)
 add_subdirectory(display)
-add_subdirectory(rpg)
-add_subdirectory(item)
 add_subdirectory(save)
 add_subdirectory(time)
 add_subdirectory(util)