fsautocomplete .net core doesnt run with just .net core 3 runtime installed

[enricosada]

fsautocomplete require a .net core runtime v2 installed.

Otherwise runnning dotnet fsautocomplete.dll results in:

It was not possible to find any compatible framework version
The specified framework 'Microsoft.NETCore.App', version '2.1.0' was not found.
  - Check application dependencies and target a framework version installed at:
      C:\dotnetcli\dotnet-sdk-3.0.100-preview3-010431-win-x64\
  - The .NET Core Runtime and SDK can be installed from:
      https://aka.ms/dotnet-download
  - The following versions are installed:
      3.0.0-preview3-27503-5 at [C:\dotnetcli\dotnet-sdk-3.0.100-preview3-010431-win-x64\shared\Microsoft.NETCore.App]
  - Installing .NET Core prerequisites might help resolve this problem:
      https://go.microsoft.com/fwlink/?linkid=798306

That's expected by .NET Core apps, because was compiled targeting 2.1 runtime, so by default as FDD will require >= 2.1 < 3 (minor and path version upgrade allowed)

A workaround can be setting the runtime option "rollForwardOnNoCandidateFx": 2 ( more info in https://natemcmaster.com/blog/2019/01/09/netcore-primitives-3/#automatically-run-on-higher-major-or-minor-versions ) so when the application is executed, it will also allow to use runtime >= 2.1

[enricosada]

ref ionide/ionide-vscode-fsharp#1057