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.

doc/site/core/class.markdown

@@ -4,3 +4,23 @@
 ### **name**
 
 The name of the class.
+
+### **supertype**
+
+The superclass of this class.
+
+    :::dart
+    class Crustacean {}
+    class Crab is Crustacean {}
+
+    IO.print(Crab.supertype)        // "Crustacean"
+
+A class with no explicit superclass implicitly inherits Object:
+
+    :::dart
+    IO.print(Crustacean.supertype)  // "Object"
+
+Object forms the root of the class hierarchy and has no supertype:
+
+    :::dart
+    IO.print(Object.supertype)  // "null"