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/animation/CMakeLists.txt

@@ -7,5 +7,3 @@ include(dusktest)
 
 # Tests
 dusktest(test_easing.c)
-dusktest(test_keyframe.c)
-dusktest(test_animation.c)