Add isDrawByFiftyMoves method (#477)

* feat(draw by fifty moves) : DONE
- create a new function to check if it's a draw by fifty moves rule or not.
- this will help UI devs to show exact reason of a draw.

* - docs(add fifty moves rule example) : DONE

---------

Co-authored-by: HarshilSySCreations <[email protected]>

Author: HarshilPatel007

Diff for: README.md

@@ -492,6 +492,16 @@ chess.isDraw()
 // -> true
 ```
 
+### .isDrawByFiftyMoves()
+
+Returns true or false if the game is drawn by 50-move rule.
+
+```ts
+const chess = new Chess('4k3/4P3/4K3/8/8/8/8/8 b - - 0 78')
+chess.isDrawByFiftyMoves()
+// -> true
+```
+
 ### .isInsufficientMaterial()
 
 Returns true if the game is drawn due to insufficient material (K vs. K, K vs.

Diff for: src/chess.ts

@@ -1086,16 +1086,20 @@ export class Chess {
     return this._getPositionCount(this.fen()) >= 3
   }
 
-  isDraw() {
+  isDrawByFiftyMoves(): boolean {
+    return this._halfMoves >= 100 // 50 moves per side = 100 half moves
+  }
+
+  isDraw(): boolean {
     return (
-      this._halfMoves >= 100 || // 50 moves per side = 100 half moves
+      this.isDrawByFiftyMoves() ||
       this.isStalemate() ||
       this.isInsufficientMaterial() ||
       this.isThreefoldRepetition()
     )
   }
 
-  isGameOver() {
+  isGameOver(): boolean {
     return this.isCheckmate() || this.isStalemate() || this.isDraw()
   }