Export types expected by the geo mark

[nshiab]

Hi!

Would it be possible to export the type expected by the geo mark?

This would be helpful when working with TypeScript.

From what I understand, we currently need to install d3-geo to access the GeoPermissible type. If this type were exported directly from plot, explicitly installing d3-geo wouldn't be necessary.

Thank you!

[Fil]

Would we use https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/geojson/index.d.ts rather than the d3 types here? (We might have to add the sphere object, but I can't really make sense of the additional definitions in GeoPermissible.)

[nshiab]

This is a great idea!