Build for net6.0 + netstandard2.0

Author: thinkbeforecoding

src/Fargo/Fargo.fsproj

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <TargetFrameworks>netstandard2.0;net6.0</TargetFrameworks>
     <Description>A functional argument parser with built in completion.</Description>
     <PackageId>Fargo.CmdLine</PackageId>
     <PublishRepositoryUrl>true</PublishRepositoryUrl>