Add support for default GeoJson style.

[hlvlad]

Hello, thanks for great extension.

Currently, if I open my GeoJson data consisting of few Polygons, they all are displayed with same fill color:

I can change their opacity or fill color manually using UI, but I have to do it each time I open new file. It would be great to have a VsCode setting which would allow me to set some default styles for my GeoJson data.

Regards, Vlad