Allow for more complex first move/castling behaviour

Ouk Chaktrang (https://www.pychess.org/variants/cambodian) is pretty interesting because it has some unique first move behaviour - the king and queen both have moves that only work as their first move. We can encode these with the `f` betza flag, but currently the logic for determining whether a first move can execute is based only on what rank the piece is on, because previously it has only been used for pawns.

To complicate things further (or simplify, idk, since castling needs a rework anyway), the first move data for these pieces is stored in the castling part of the FEN, at least in fairy/pychess.

So we need:
* More flexible methods for importing/exporting FEN strings
* More flexible first move logic
* Also a way to remove first move flags with the indirect rook attack that Ouk Chaktrang has, but this can be done with actions