Include square to board() output

[fitztrev]

This PR adds the square notation to the board() output.

I was looking for an easy way to find all the squares that a certain type of piece would be on.

[jhlywa]

@fitztrev This change looks reasonable and should be backwards compatible with any existing code that uses the .board() function. The tests, however, appear to be failing in CI though. Can you tweak the tests and docs before I merge? Thanks

[fitztrev]

@jhlywa Absolutely!

• Fixed tests
• Updated README docs with new .board() output

[jhlywa]

Thanks Trevor!