[position] Rename the board crate → chessfriend_position in Cargo.toml

Rename the crate, but don't move any files.

explorer/src/main.rs

@@ -1,6 +1,5 @@
-use board::piece::{Color, Piece, Shape};
-use board::Position;
 use chessfriend_core::Square;
+use chessfriend_position::{fen::ToFen, MakeMoveBuilder, MoveBuilder, Position, PositionBuilder};
 use clap::{Arg, Command};
 use rustyline::error::ReadlineError;
 use rustyline::DefaultEditor;