Core: Stillbirth/Miscarriage mechanic

[schwarherz]

Create a mechanic for dealing with failed pregnancies. Do not use the NDefines.NCharacter.INFANT_DEATH_CHANCE define from Paradox as it causes CTDs and generates characters that do nothing but waste computer space. Instead use the cancel_pregnancy = yes effect in an event to prevent them from existing in the first place.

Should be an approx. 30% chance for miscarriage and approx 1-3% chance for stillbirth.