Add Support for Path.GetRelativePath

[joostas]

Please, add support for
public static string GetRelativePath (string relativeTo, string path);
which appeared in .NET Core 2.0.

[fgreinacher]

According to https://apisof.net/catalog/System.IO.Path.GetRelativePath(String,String) this is exposed in .NET Standard 2.1, so we'd have to introduce a new TFM. Whenever we do that we should probably do an API diff between S.IO.A and S.IO to see what else is missing.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[fgreinacher]

This is much easier to implement now that we only compile against two target frameworks