Add .Net Core 2 as a target framework

[ruimaciel]

When GeoJSON4EntityFramework is added to a .Net Core 2.2 project, the following warnings are shown:

/home/rui/Test/Test.csproj : warning NU1701: Package 'GeoJSON4EntityFramework 2.2.0' was restored using '.NETFramework,Version=v4.6.1' instead of the project target framework '.NETCoreApp,Version=v2.2'. This package may not be fully compatible with your project.

It would be nice of the project explicitly targeted the .Net Core framework.

[dupski]

As an alternative for .Net Core you can use NetTopologySuite.IO.GeoJSON

[alatas]

@ruimaciel This is a solid requirements that I'm aware of. But, in the meantime, the source and dependencies of this project are not suitable porting in a short period of time. I am planning to design and implement a new v3 branch which is targeting .net standard, and keep v2 in the .net 4.x.

I assume that this new branch requires lots of change. So that, I am considering a new implementation from ground up with C# and .net standard targeting. It will be a lot faster and easier method to retarget this library.