Support Caïssa Britannia

[PraseodymiumSpike]

I think Caïssa Britannia would be a good variant to work on. I think that the only two things that need to be added are ones that make a lot of sense to add. They are:

• diagonal facing royals
• sliding royals cannot castle or move through check, facing royals, or diagonal facing royals

[ianfab]

Rules: https://www.chessvariants.com/large.dir/british.html