Support .net Core 3.0

[baronfel]

.net core global tools have to opt-in to support for other major versions of the .net core runtime. This can be done by compiling for netcoreapp3.0 as another target framework (possibly difficult), or by adding a runtimeconfig.template.json to the same directory as your dotnet-global-tool project (see FAKE for an example).

[isaacabraham]

@baronfel yes, and ideally latest version of F# compiler - looks like things like implicit yield aren't supported