document and test GeoJSON.read(path)

[visr]

Fixes #49.

This needs a JSON3 release with quinnj/JSON3.jl#243, so I require that in the compat.

One thing I don't like so much about this is the error message if you try to read a file that doesn't exist, but that's really a JSON3 issue, and a consequence of tring to figure out if a string represents a path or JSON. This is the error you get:

ERROR: ArgumentError: invalid JSON at byte position 1 while parsing type GeoJSON.GeoJSONT{3, Float32}: ExpectedOpeningObjectChar
your.geojson