Expose skeletal joints to frontend hierarchy (#56)

This PR exposes skeletal joint entities to the frontend hierarchy,
allowing the user to pose rigged models by selecting the joints and
transforming them.

Additionally, entity-type information (e.g., Mesh, Skeleton, Bone,
Light, etc.) has been added to the data structure passed to the
frontend, which is rendered with icons in the tree view.

utility/src/lib/match.ts

@@ -83,7 +83,7 @@ export function match<K extends Key, T extends Matcher<K>>(
 	subject: K | null | undefined,
 	matcher: T,
 ): MatchReturn<K, T> {
-	if (subject && subject in matcher)
+	if (subject != null && subject in matcher)
 		return matcher[subject]!();
 
 	if ("_" in matcher)