Add definitions for CLASS_BIRTH_EVENT and CLASS_NO_BIRTH_EVENT

[yiwen-h]

It's very complicated - how to fit it into the current classification register format?

https://github.com/The-Strategy-Unit/nhp_data/blob/38c68b59715daea218f4eea1e00cd381bbd41cb8/src/nhp/data/raw_data/inpatients.py#L18