refactor: split class creation into raw single-class and superclass binding phases

The bootstrapping of Object and Class now uses a two-phase approach: first creating a raw class with no metaclass or superclass via wrenNewSingleClass, then explicitly wiring superclass relationships with wrenBindSuperclass. This eliminates the need for special-case handling of null superclasses in the old newClass function and makes the metaclass chain setup explicit in wrenInitializeCore.

test/class/type.wren

@@ -0,0 +1,10 @@
+class Foo {}
+
+// A class is a class.
+IO.write(Foo is Class) // expect: true
+
+// It's metatype is also a class.
+IO.write(Foo.type is Class) // expect: true
+
+// The metatype's metatype is Class.
+IO.write(Foo.type.type == Class) // expect: true