Remove the editor feature

It's not exactly complete, but an additional maintenance burden. Better make this a separate component, should we need it again in the future. But instead, I strongly recommend using a real GIS software like QGIS to edit the data, and not try to reimplement some of its features.

This completely removes the editor (#3 (closed)) from the project. There's also the edit-props branch (current tip being a5090597), which contains work that has been started to give the editor a way to edit GeoJSON properties. That branch has not been finished, but its commit a5090597, which defines a npm package, has been cherry-picked as 1972e673 into this one, since it's not exactly related to the editor feature.