stonecutter: setup project for multiversion support

- Switch to Stonecutter build system
- Migrate mappings to Mojmap

src/main/resources/puzzle-splashscreen_neoforge.mixins.json

@@ -0,0 +1,11 @@
+{
+  "required": true,
+  "package": "net.puzzlemc.splashscreen.mixin",
+  "compatibilityLevel": "JAVA_17",
+  "client": [
+    "MixinNeoForgeLoadingOverlay"
+  ],
+  "injectors": {
+    "defaultRequire": 1
+  }
+}