Add JOSM compatibility for geojson import/export

[fallingcats]

JOSM, the Java OpenStreetMap Editor is a great performant tool to edit geojson maps.
Unfortunately it's bit of a lossy process going from om-gui -> josm -> om-gui, so here is the stuff to make it work anyway.

If anyone wants to try it, here is what you need to know:

• Obstacles need to have the same index property set as their associated area
• index needs to be unique (not necessarily consecutive - reordered on upload); mowing and navigation both start at 0.

To make editing easier, omit the id property and instead specify type as area or navigation otherwise obstacle is assumed.

[fallingcats]

I could also go ahead and remove the "backup id" again since it's not strictly necessary, but keeping it ensures nothing changes between uploads of the same file. I don't have strong feelings either way.

[fallingcats]

Sorry but I have no idea what to do about the gh actions failing. Apparently they've deprecated {up,down}load-artifact@v3 - see https://github.com/orgs/community/discussions/142581#discussion-7367108

I tried, but I don't really understand actions enough to fix that.