Implements resize observer to call invalidateSize on map when container is resized

[liqueurdetoile]

Following issue #39, here's a proposal aiming to implement resizeObserver on map container to trigger invalidateSize on size when map container is programmatically resized after initialization.

Successfully tested with Hestia theme on Chrome 96, Firefox 95 and Edge 96.

[goiblas]

Great contribution, thx