Capture initial resize event

[birkskyum]

Closes #2551

Captures the initial resize event from the resize observer, so it's more similar to the previous implementation that didn't fire on setup.

Adds the browser test that checks that the 'load' event comes before 'resize' and 'moveend'

Launch Checklist

• Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!

[HarelM]

Seems like our unit tests are covering is 😀
I guess there's a need to add another call to trigger the observer. But this test can remain to make sure resize doesn't get called on first event.

[birkskyum]

Updated the unit test to observe twice

[HarelM]

Can you add another test or and expectations between the first and second callback?

[birkskyum]

Great idea, I added these so it now says:

            // The initial "observe" event fired by ResizeObserver should be captured/muted
            // in the map constructor

            observerCallback();
            expect(spyA).not.toHaveBeenCalled();
            expect(spyB).not.toHaveBeenCalled();

            // Following "observe" events should fire a resize / _update

            observerCallback();
            expect(spyA).toHaveBeenCalled();
            expect(spyB).toHaveBeenCalled();
            ```