Prefer const vs let for variables that are only declared once

Requires that variable declarations use `const` instead of `let` and `var` if possible.

This is mostly for code readability, performance wins are debatable.

src/Viewport.ts

@@ -180,7 +180,7 @@ export class Viewport implements IViewport {
    * @param ev The touch event.
    */
   public onTouchMove(ev: TouchEvent): void {
-    let deltaY = this._lastTouchY - ev.touches[0].pageY;
+    const deltaY = this._lastTouchY - ev.touches[0].pageY;
     this._lastTouchY = ev.touches[0].pageY;
     if (deltaY === 0) {
       return;