Add API for allowing the map to zoom out to z0

Nobody has claimed this yet.

## Motivation

https://github.com/mapbox/mapbox-gl-js/pull/6918 clamps latitude values to +-85.56° to prevent `fitBounds` values such as -90 from throwing an error. This inadvertently made zooming all the way out to z0 impossible. This should have been considered a breaking change. At least one user has already asked about this. Clamping latitudes to valid Web Mercator values is the correct behavior, but we should provide a simple API to allow a user to revert to the pre-v0.48.0 behavior.

## Design Alternatives

1. Do nothing. It can be argued that the previous behavior was a bug and we shouldn't add a way to reintroduce "buggy" behavior. `map.transform.latRange = null` achieves the desired effect without introducing a new API

2. `map.setMaxBounds(null);` used to achieve this functionality so we could ensure that this function call sets the `latRange` to `null`

3. We could add a map option such as `clampLatRange` with the default `true`

4. We could have an explicit `transform` method such as `map.transform.unclampLatRange()`.

## Design

The more I think about it, I might lean towards option 1. There's a way to achieve this without introducing a new API that allows arguably undesirable behavior. If we do want to implement this, I think 3 might be the simplest way for a user to allow this behavior.

Start by reviewing the map.transform.latRange behavior and the existing setMaxBounds(null) path described in the issue. Compare the four listed design alternatives and determine which API behavior should be supported; the work is done when the chosen approach consistently permits or rejects zooming to z0 as specified.

Written by the indexing model from the issue text.