Add constrainResolution option

Sometimes we may have the functionality to set scale/resolution provided by a user and it can be any number (so constrainResolution in View class is not useful), but we would like to have constrainResolution in mouse wheel zoom event.

src/ol/interaction/MouseWheelZoom.js

@@ -29,6 +29,9 @@ export const Mode = {
  * @property {boolean} [useAnchor=true] Enable zooming using the mouse's
  * location as the anchor. When set to `false`, zooming in and out will zoom to
  * the center of the screen instead of zooming on the mouse's location.
+ * @property {boolean} [constrainResolution=false] If true, the mouse wheel zoom
+ * event will always animate to the closest zoom level after an interaction;
+ * false means intermediary zoom levels are allowed.
  */
 
 /**
@@ -84,6 +87,12 @@ class MouseWheelZoom extends Interaction {
     this.useAnchor_ =
       options.useAnchor !== undefined ? options.useAnchor : true;
 
+    /**
+     * @private
+     * @type {boolean}
+     */
+    this.constrainResolution_ = options.constrainResolution !== undefined ? options.constrainResolution : false;
+    
     /**
      * @private
      * @type {import("../events/condition.js").Condition}
@@ -214,7 +223,7 @@ class MouseWheelZoom extends Interaction {
     }
 
     const view = map.getView();
-    if (this.mode_ === Mode.TRACKPAD && !view.getConstrainResolution()) {
+    if (this.mode_ === Mode.TRACKPAD && !(view.getConstrainResolution() || this.constrainResolution_)) {
       if (this.trackpadTimeoutId_) {
         clearTimeout(this.trackpadTimeoutId_);
       } else {
@@ -260,7 +269,7 @@ class MouseWheelZoom extends Interaction {
         -this.maxDelta_ * this.deltaPerZoom_,
         this.maxDelta_ * this.deltaPerZoom_
       ) / this.deltaPerZoom_;
-    if (view.getConstrainResolution()) {
+    if (view.getConstrainResolution() || this.constrainResolution_) {
       // view has a zoom constraint, zoom by 1
       delta = delta ? (delta > 0 ? 1 : -1) : 0;
     }