Add the ability to render <map-featurecaption> as the feature label via "CSS"

[prushforth]

There could be two "modes" to this:

• as a single un-cut <map-feature> that requires to be accessibly labelled on the child <map-geometry> according to an algorithm that should be selectable and responsive
• as a tiled feature that is represented across multiple tiles, but still requires to be accessibly labelled, but maybe the renderer should be able to re-compose the part of the original geometry that is represented in tiled parts and perform the labelling algorithm on the resulting geometry similar to how it would proceed in the first case. If unique <map-feature>s were identified by some special attribute e.g. data-fid, the value of the attribute could be used to merge related geometries prior to running the label placement algorithm. Currently, the tiled feature geometry is rendered and thrown away, which might still be ok after the labelling had been performed, tbd.

[prushforth]

Closing, duplicate of #947