[WIP] Ground overlays

[tmcw]

Work-in-progress!

Support for ground overlays. This may not land, it's still an experiment.

Concrete TODOs:

• Support groundoverlays in the "folders" mode
• Documentation (update readme, add to jsdoc)
• Support rotation
• Differentiate ground overlays from other types
• Writeup examples of how to actually integrate this with Leaflet & Mapbox GL / Maplibre

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Published as a prerelease - @tmcw/togeojson@3.1.0-0

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Less-concrete TODOs:

• Should ground overlays even be features? In the current code, an overlay becomes a polygon with an icon href property. This is a little weird! Implementations will, most likely, pick these overlays out and display them in a totally different manner than features. Showing an overlay as a box is probably not intended. On the other hand, if they aren't features then they'll not be included in the featurecollection returned by kml(), and will lead to more format fracturing and invention.