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]>

.github/workflows/test.yml

@@ -32,4 +32,4 @@ jobs:
             git \
             libssl-dev
       - name: Run tests
-        run: ./scripts/test-linux.sh
+        run: ./scripts/linux/test-linux.sh