dynamic popup anchor with a non bottom preference

The [Popup `anchor` option](https://www.mapbox.com/mapbox-gl-js/api/#popup) is documented as

> A string indicating the part of the Popup that should be positioned closest to the coordinate set via Popup#setLngLat . Options are 'center' , 'top' , 'bottom' , 'left' , 'right' , 'top-left' , 'top-right' , 'bottom-left' , and 'bottom-right' . If unset the anchor will be dynamically set to ensure the popup falls within the map container **with a preference for 'bottom' .**

I'd like to retain the dynamic feature to ensure the popup doesn't fall off the map, while setting a preference other than `bottom`.

I think the best solution is to change the definition of the `anchor` option to be the preferred anchor and introduce a new `fixedAnchor` option to tell if you want it to avoid falling of the map or not.

This is a backwards incompatible change, but I think any change made in a backwards compatible way would introduce to much complexity to the API (for example leaving anchor as is and adding a new `anchorPreference` option).

I don't mind trying to do a PR, but what do people think about this breaking change?

Review the Popup `anchor` option documentation and `Popup#setLngLat` behavior, then inspect linked pull request #11055. The work is complete when dynamic positioning supports a preference other than `bottom` without allowing the popup to fall outside the map container, with the resulting API behavior documented.

Written by the indexing model from the issue text.