refactor: extract validateSuperclass helper and move subclassing tests to inheritance directory

test/inheritance/inherit_from_closure.wren

@@ -0,0 +1,8 @@
+var ClosureType
+
+{
+  var a = "a"
+  ClosureType = new Fn { IO.print(a) }.type
+}
+
+class Subclass is ClosureType {} // expect runtime error: Subclass cannot inherit from Fn.