Add support for fitBounds and fitScreenCoordinates on globe

[karimnaaji]

This PR adds support for fitBounds and fitScreenCoordinates with globe projection by leveraging the method introduced in #12138:

• When calling fitBounds or fitScreenCoordinates, redirect all internal calls to _cameraForBoxAndBearing to the newly introduced method from Add support for cameraForBounds on globe projection #12138 instead of the iterative approach performing frustum intersection by raymarching steps (which isn't adapted for globe).
• Remove the iterative method as it's now unused
• Remove warnings about unsupported API and update the docs
• Account for terrain directly in _cameraForBounds in order to be on par with previous behavior from the iterative frustum approach.

Fixes https://github.com/mapbox/gl-js-team/issues/502

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• manually test the debug page
• tagged @mapbox/gl-native if this PR includes shader changes or needs a native port
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Add support for fitBounds and fitScreenCoordinates with globe projection</changelog>

[SnailBones]

LGTM! Just a few small nits.