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/api/returns.c

@@ -27,7 +27,7 @@ static void returnFalse(WrenVM* vm)
   wrenReturnBool(vm, false);
 }
 
-WrenForeignMethodFn returnsBindForeign(const char* signature)
+WrenForeignMethodFn returnsBindMethod(const char* signature)
 {
   if (strcmp(signature, "static Api.implicitNull") == 0) return implicitNull;
   if (strcmp(signature, "static Api.returnInt") == 0) return returnInt;