Add a "popup.open" event

[jmeyers91]

Basically the same as Add a "popup.close" event except an open event should also trigger when the popup opens.

Expected Behavior

This should log 'open' to the console when the popup opens:

let popup = new mapboxgl.Popup()
popup.on('open', function(e) {
    console.log('open');
})

Actual Behavior

The open event is never fired.

[jfirebaugh]

Hey @jmeyers91! This would make a good first contribution. Would you like to take a stab at a PR?

[jmeyers91]

Sure

[jfirebaugh]

Done in #6311. Thanks @jmeyers91!