feat: prevent subclassing of built-in types with runtime error in class creation

Add a check in the interpreter's class creation path that detects when a user-defined class attempts to inherit from any of the seven built-in types (Class, Fiber, Fn, List, Map, Range, String). When such an inheritance is detected, a runtime error is raised with a descriptive message indicating the class name and that it may not subclass a built-in. This implements the restriction discussed in issue #70.

Also adds test files for each built-in type to verify the runtime error is correctly triggered, and includes a commented-out test for closure subclassing that currently causes a segfault.

test/map/subclassing_map.wren

@@ -0,0 +1,2 @@
+class SubMap is Map {} // expect runtime error: Class 'SubMap' may not subclass a built-in
+