Expose API via useImperativeHandle.

[isofarro]

Following on from the suggestion in #14 , reimplemented my changes using a forwardRef and useImperativeHandle. Returns an instance of the Chessground API for the current board.

Sample code demonstrating it's use (added to the README too):

import { useEffect, useRef } from "react";
import Chessground, { Api, Config, Key } from "@react-chess/chessground";

// these styles must be imported somewhere
import "chessground/assets/chessground.base.css";
import "chessground/assets/chessground.brown.css";
import "chessground/assets/chessground.cburnett.css";

const CONFIG: Config = {
  movable: { free: false },
};

// Demo game moves in long algebraic form
const MOVES = (
  "e2e4 e7e5 g1f3 d7d6 d2d4 c8g4 d4e5 g4f3 d1f3 d6e5 " +
  "f1c4 g8f6 f3b3 d8e7 b1c3 c7c6 c1g5 b7b5 c3b5 c6b5 " +
  "c4b5 b8d7 e1c1 a8d8 d1d7 d8d7 h1d1 e7e6 b5d7 f6d7 " +
  "b3b8 d7b8 d1d8"
).split(" ");

export const DemoGameMoves = () => {
  const apiRef = useRef<Api | undefined>();

  useEffect(() => {
    // Make a move every 2 seconds
    const interval = setInterval(() => {
      const move = MOVES.shift();
      if (move) {
        apiRef.current!.move(move.substring(0,2) as Key, move.substring(2,4) as Key);
      } else {
        clearInterval(interval);
      }
    }, 2000);
    return () => clearInterval(interval);
  });

  return (
    <Chessground
        width={640} height={640}
        config={CONFIG} ref={apiRef}
    />
  );
};

[nowszy94]

Would be nice to have api usage example in README.md

[isofarro]

Would be nice to have api usage example in README.md

I've added the example demo-game component from this PR description to the README, and added ref to the props documentation.