[experiment] add SourceLink to the Fable.Remoting.Json nuget package

[Numpsy]

I saw #262 and thought that as I've been having a go with Remoting recently and SourceLink might be useful, Id have a go at doing the change.

It's labelled 'experiment' because I've never done it on a project using Paket before - I gather from reading elsewhere that the 'copy_local: true' is the way to make it attach SourceLink to the build, but to not make the produced Remoting package have a runtime dependency on the SourceLink packages. So - I wanted to confirm that is ok before doing any of the other packages.

This is what the before (the current package published to nuget.org on the left) and after (a local package build on the right):

So the repository information is there.

Note: This is using the SourceLink approach of putting the pdb files in the nupkg - an alternative is to make a snupkg file, but I haven't actually done that before, and don't know what the publishing procedure is.

Related note: There are warnings in the package build about PackageLicenseUrl being deprecated - any thoughts or objections for a PR to replace it with PackageLicenseExpression?

[Numpsy]

On a semi related note, I tried to get my local build marked as deterministic as well, but didn't get it to work (that may or may not be down to dotnet/fsharp#11920, my knowledge of the tools is a bit too thin to be certain)

[Zaid-Ajaj]

Since I don't really know how to test this and you are not sure it will work correctly, I will just close it for the time being.