Add VectorGrid support

[blackary]

Fixes #107

[kotfic]

Tested locally, getting good vector tiles. Thanks!

[kotfic]

FWIW setting "interactive": True on the VectorGridProtobuf layer causes errors due to an upstream bug Leaflet/Leaflet.VectorGrid#273 Not sure there is anything to do about it.

[randyzwitch]

This has a merge conflict now @blackary

[blackary]

@kotfic If the bug is on the underlying Leaflet library, it will likely be difficult to fix in this package (not necessarily impossible, but much more difficult, since this is two layers above leaflet -- leaflet -> folium -> streamlit-folium). Do you have an example streamlit script which shows the issue?

[kotfic]

@blackary yeah sorry not suggesting any action item on this PR. There are 4 or 5 issues on Leaflet.VectorGrid describing the problem, seeing as the last real commit on that library was July 2019 I don't have much hope of it being fixed. Its just a bummer that it has the function (so the duck typing passes) but the function is broken. It is equally a bummer that leaflet/javascript doesn't support determining the layer type so onLayerClick can't easily skip implement a code path that avoids the exception.