Implement selected piece highlighting

Issue #3

frontend/visuals.js

@@ -86,6 +86,23 @@ export function rowColToBoardIndex(row, col) {
   return `${colLetter}${rowDigit}`;
 }
 
+export class ActiveSquares {
+  constructor() {
+    this.selectedSquare = null;
+  }
+  selectSquare(canvas, position) {
+    let [r, c] = boardIndexToRowCol(position);
+    canvas.drawSquareColor("#c0c0ff", r, c);
+    this.selectedSquare = position;
+  }
+  unselectSquare(canvas) {
+    if (this.selectedSquare === null) return;
+    let [r, c] = boardIndexToRowCol(this.selectedSquare);
+    canvas.drawSquareColor(canvas.decideColor(r, c), r, c);
+    this.selectedSquare = null;
+  }
+}
+
 class PieceVis {
   constructor(piece) {
     this.piece = piece;