refactor(cubed): migrate block definitions to JSON

Move block data from TOML to JSON under assets/cubed/blocks, add registry.json for block and item IDs, and relocate textures and sounds to the cubed namespace.

assets/cubed/blocks/air.json

@@ -0,0 +1,14 @@
+{
+    "name": "air",
+    "properties": {
+        "is_liquid": false,
+        "is_cross_plane": false,
+        "is_transparent": true,
+        "is_passable": true,
+        "is_discard": true,
+        "is_blend": false,
+        "is_transitional": false,
+        "is_gas": true,
+        "roughness": 1.0
+    }
+}