feat: add wrenReturnBool API for boolean return from foreign calls

Add a new public API function `wrenReturnBool` to the Wren C API, enabling
foreign method implementations to return boolean values. The implementation
stores the boolean as a Wren BOOL_VAL in the foreign call slot and clears
the slot pointer, following the same pattern as existing `wrenReturnString`.

Also add the missing `prep` target to the .PHONY list in the Makefile and
include `<stdbool.h>` in the public header to support the `bool` type used
in the new function signature.

src/wren_vm.c

@@ -1236,3 +1236,11 @@ void wrenReturnString(WrenVM* vm, const char* text, int length)
   *vm->foreignCallSlot = wrenNewString(vm, text, size);
   vm->foreignCallSlot = NULL;
 }
+
+void wrenReturnBool(WrenVM* vm, bool value)
+{
+  ASSERT(vm->foreignCallSlot != NULL, "Must be in foreign call.");
+
+  *vm->foreignCallSlot = BOOL_VAL(value);
+  vm->foreignCallSlot = NULL;
+}