Instantiate chessboard with FEN string

Why?
This would make test code much nicer when setting up specific position.
It also allows setting up the board on Lichess, then simply "importing" it.

Where?
Probably just directly in the Chessboard class.

Documentation: [https://www.chess.com/terms/fen-chess](https://www.chess.com/terms/fen-chess)

Start by reading the Chessboard class and the linked FEN specification to understand the board state it currently constructs. Define how a FEN string should initialize a position, then verify that a board created from a representative FEN matches the described pieces and state; the issue does not name an existing test file.

Written by the indexing model from the issue text.