Add support for netcoreapp3.0

[baronfel]

This tool will fail to run on .net core 3 runtimes, because the default roll-forward config only allows rolling-forward across minor versions. There are a few options here, in order from worst to best:

• let the user specify a roll-forward env variable: DOTNET_ROLL_FORWARD_ON_NO_CANDIDATE_FX=2 to allow major-version roll forward
• add a matching setting to this tool's runtimeconfig.json file, like I did in Paket: add roll-forward config to enable running on later major versions of the runtime fsprojects/Paket#3635
• actually do the work to multitarget the tool to netcoreapp3.0