Implementation of the en passant rule

[turtureanu]

Currently, Kung Fu Chess doesn't seem to allow en passant.

To replicate:

• Go to https://www.kfchess.com/
• Click on "Play vs Friend"
• Play against yourself until you get a position in which you should be able to play en passant.

For example:

1. e4 e6
2. e5 d5
3. exd6 e.p.

[chess123mate]

I suggest en passant should only be allowed while the cooldown of the pawn (that can be captured) is active