Request change to MPFVariable to support up to 9 Players

[founfabug]

Doco at https://missionpinball.org/latest/gmc/reference/mpf-variable/ suggests:
MPFVariable currently supports a maximum of four players for player-specific variables. If you need more than this, please let us know!

I have an existing 9 player design utilising mc. This is to accommodate groups of young novices. In this use-case the desire for group participation, and short ball-in-play times make larger player numbers desirable.