feat: add foreign class support with allocation, construct opcodes, and CLI callback wiring

Implement the initial infrastructure for foreign classes in the Wren VM, including new opcodes (FOREIGN_CONSTRUCT, FOREIGN_CLASS), a WrenBindForeignClassFn callback type in the public API, and updated CLI vm.c to store and forward foreign binding callbacks via setForeignCallbacks(). The compiler now tracks whether a class is foreign via a new isForeign flag in ClassCompiler, emits CODE_FOREIGN_CONSTRUCT instead of CODE_CONSTRUCT for foreign class constructors, and errors on field definitions inside foreign classes. The debug dump and opcode header gain entries for the new opcodes, and wrenBindSuperclass handles the -1 numFields sentinel for foreign classes to prevent field inheritance from superclasses.

test/language/inheritance/inherit_from_nonclass.wren

@@ -1 +1 @@
-class Foo is 123 {} // expect runtime error: Must inherit from a class.
+class Foo is 123 {} // expect runtime error: Class 'Foo' cannot inherit from a non-class object.