.NET 5 Support

[NatElkins]

Is your feature request related to a problem? Please describe.
Not specifically.

Describe the solution you'd like
.NET 5 support, and the ability to reference Nuget packages from script files. I also think this is one way to handle themes (just package it up as a Nuget package).

Describe alternatives you've considered

Additional context

I'm mostly just creating this issue to track. I'll try and find some time to take a crack at some point.

[baronfel]

Brief punch list to update to .net 5 and reference nuget packages would be:

• Update to FCS 38
• Add a required reference to Microsoft.Build.Utilities.Core (IIRC, might need to check FSAC for this but it's a nuget dep that's not in the FCS nuspec right now)
• Add a package reference to FSharp.DependencyManager.Nuget to get the nuget #r support implicitly, since the dll for that will be co-located with the FCS dll when you run fornax.

[baronfel]

done in #78