Create separate project for .NET 5 support with updated dependencies/targets netstandard2.0

I'd like to use breeze in .NET 5 world. To avoid version mismatches, etc. we can update dependencies and targets and I think it'll work.

I don't know what sort of publishing infrastructure or testing will be involved, but putting this PR out there to hopefully kickstart this effort.

Please let me know if there's anything else I can do to move this forward.