Add proficiency bonus to monster statblocks

[MurdoMaclachlan]

This PR follows the proposed solution in https://github.com/naturalcrit/homebrewery/issues/3397.

I've given the proficiency bonus a right-padding of 0.5em, which about mimics the positioning in Monsters of the Multiverse, and I have it randomly choosing between 2 and 6 for the bonus value, since that's the usual range.