make sure 'new GridStack(el)' set el.gridstack=this

right away as code during loading might depend on those vars being set.

Author: adumesny

src/gridstack.ts

@@ -267,6 +267,7 @@ export class GridStack {
    * @param opts
    */
   public constructor(el: GridHTMLElement, opts: GridStackOptions = {}) {
+    el.gridstack = this;
     this.el = el; // exposed HTML element to the user
     opts = opts || {}; // handles null/undefined/0