Add Polygon to evaluable geometry for within expression

[markusand]

Motivation

The within expression simplified a lot the visualization of features depending on their geometry, but only Point and LineString are supported (according to docs). I would find very useful to support Polygon as well, specially as I'm working with polygon cells.

It may be a discussion of what whitin means for polygon (fully contained, some intersection, ...) but that is the same case with LineString, isn't it? I would consider a Polygon intersecting another one as within

Implementation

Polygon contour can be considered a LineString, so I guess it would be quite straightforward to perform the operation.
This change would not affect the public API of MapboxGL.

[asheemmamoowala]

Closing as duplicate of #9795 to keep the discussion in one place. @markusand Thanks for also commenting there.