Add Class.supertype.

Thanks, Michel!

test/class/supertype.wren

@@ -0,0 +1,15 @@
+class Foo {}
+
+class Bar is Foo {}
+
+class Baz is Bar {}
+
+// A class with no explicit superclass inherits Object.
+IO.print(Foo.supertype == Object) // expect: true
+
+// Otherwise, it's the superclass.
+IO.print(Bar.supertype == Foo) // expect: true
+IO.print(Baz.supertype == Bar) // expect: true
+
+// Object has no supertype.
+IO.print(Object.supertype) // expect: null