Permit `padding` in transitions

[harto]

Mapbox GL JS 1.9 added support for padding in camera functions flyTo, jumpTo, etc.: mapbox/mapbox-gl-js#8638.

It would be great if a similar option was made available through this library. Our use case is that we need to adjust the target viewport based on the width of a sidebar.

[Pessimistress]

This is already supported in WebMercatorViewport.fitBounds

[mayteio]

See #1099 (comment) for a function that returns a new viewport with padding, accounting for edge cases.