[FIX] web: properly define at method in array polyfill

Commit [1] introduced a fix for old browsers to make the `at` method of
the Array class. It did so by simply adding a property on the prototype
of the Array class.
Unfortunately this way of adding a method means that it is exposed in
`for...in` loops.

To avoid this, this commit uses Object.defineProperty and sets the
property as not enumerable, removing it from the for..in loop.

[1]: https://github.com/odoo/odoo/commit/6e39e31030c28a8eb09ddcd93ca9ca382b4f6f10

opw-3824593

closes odoo/odoo#161891

Signed-off-by: Romain Derie (rde) <rde@odoo.com>

addons/web/static/src/polyfills/array.js

@@ -1,8 +1,11 @@
 if (!Array.prototype.at) {
-    Array.prototype.at = function (index) {
-        if (index >= 0) {
-            return this[index];
+    Object.defineProperty(Array.prototype, "at", {
+        enumerable: false,
+        value: function (index) {
+            if (index >= 0) {
+                return this[index];
+            }
+            return this[this.length + index];
         }
-        return this[this.length + index];
-    };
+    });
 }