Add a conversion from Vec<Feature> to GeoJson

[dabreegster]

• [X] I agree to follow the project's code of conduct.

• [X] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

As discussed in Discord, this is more convenient than let gj = geojson::GeoJson::from(features.into_iter().collect::<geojson::FeatureCollection>()); or let gj:GeoJson = FeatureCollection::from_iter(vec_of_geojson_features.into_iter()).into(); or similar

[bors[bot]]

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