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/class/subclassing_class.wren

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

test/closure/subclassing_closure.wren

@@ -0,0 +1,16 @@
+var f = null
+
+{
+  var a = "a"
+  f = new Fn {
+    IO.print(a)
+  }
+}
+//@FIXME: repair the segfault create a propper test for the subclassing
+
+//var closureSegfault = f
+//class SubClosureSegfault is closureSegfault {} 
+
+//var closureOk = f.call
+//class SubClosureOk is closureOk {} 
+

test/fiber/subclassing_fiber.wren

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

test/function/subclassing_fn.wren

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

test/list/subclassing_list.wren

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

test/map/subclassing_map.wren

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

test/range/subclassing_range.wren

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

test/string/subclassing_string.wren

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