feat: add Class.supertype primitive and documentation for retrieving superclass

Implement a new `supertype` property on Class objects that returns the superclass of a given class. The primitive `class_supertype` checks if the class has a superclass (returning null for Object) and returns the superclass object. Includes documentation in the class reference and a test file verifying behavior for explicit superclasses, implicit Object inheritance, and Object's null supertype.

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