Add back PYBIND11_HAS_INTERNALS_WITH_SMART_HOLDER_SUPPORT

This define was
* introduced with #5286
* removed with #5531

It is has been in use here:
* https://github.com/pybind/pybind11_protobuf/blob/f02a2b7653bc50eb5119d125842a3870db95d251/pybind11_protobuf/native_proto_caster.h#L89-L101

Currently pybind11 unit tests for the two holder caster backwards compatibility traits

* `copyable_holder_caster_shared_ptr_with_smart_holder_support_enabled`
* `move_only_holder_caster_unique_ptr_with_smart_holder_support_enabled`

are missing.

include/pybind11/detail/internals.h

@@ -211,6 +211,9 @@ struct internals {
     }
 };
 
+// For backwards compatibility (i.e. #ifdef guards):
+#define PYBIND11_HAS_INTERNALS_WITH_SMART_HOLDER_SUPPORT
+
 enum class holder_enum_t : uint8_t {
     undefined,
     std_unique_ptr, // Default, lacking interop with std::shared_ptr.