Change all GeoJSON Features to be GeoJSON compliant

[dopenguin]

See #479 (comment) for more information

[oeninghe-dataport]

Copied the text from there, as the linking works not-so-well with closed conversations.

While we're here, we may change this to become GeoJSON-compliant.

While it's allowed to add a title as a foreign member, we are not allowed to change the type. Instead of adding 'reverse_geocoded' as type, we may just introduce a new field 'source' as another foreign member to keep the 'type' clean. This is especially relevant if/when our results are used outside our own sphere.

src/core/types/utils.ts

export interface PolarGeoJsonFeature<
	G extends UsedGeometry | null = UsedGeometry,
	P = GeoJsonProperties,
extends Omit<Feature<G, P>, 'type'> {