Expose the stableSort function

This is a low level function, essential to OpenLayers so it is always
shipped with OpenLayers.

src/ol/array.js

@@ -196,6 +196,7 @@ export function equals(arr1, arr2) {
 /**
  * @param {Array<*>} arr The array to sort (modifies original).
  * @param {Function} compareFnc Comparison function.
+ * @api
  */
 export function stableSort(arr, compareFnc) {
   const length = arr.length;