Add 'idle' event: fires when no further rendering is expected without further interaction.

[ChrisLoer]

We fairly frequently receive requests for a map event that fires when the map is "done rendering". This is a kind of squishy concept because so many things can cause the map to change. Even if there's no user interaction with the map, tiles can update at any time and trigger map updates. However, we have a pretty good idea of when we're entering an idle state:

• All requested tiles are loaded
• No transitions are in progress
• No camera animations are in progress

This PR fires an idle event in that condition. One reason someone might want to use this event is to make changes to the map, and then do a canvas capture after everything has finished rendering.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page
• tagged @mapbox/studio and/or @mapbox/maps-design if this PR includes style spec changes

cc @kkaefer @1ec5 @tmpsantos @mourner @ryanhamley @mollymerp

[ryanhamley]

🎉 looks good

[mollymerp]

Prior art #2471

I do think this kind of event is something we should add to the library – we've previously discussed calling it renderend or renderstable. My vote is for renderend because of the precedent of zoomend etc.