add dependency to FSharp.Core

ensure there are no runtime errors when FSharp.Core cannot be located:
* add reference to FSharp.Core 4.7.1 in Flow.Launcher.Core
* update FSharp.Core to 4.7.1 in HelloWorldFSharp

Author: JohnTheGr8

Flow.Launcher.Core/Flow.Launcher.Core.csproj

@@ -52,6 +52,7 @@
   </ItemGroup>
 
   <ItemGroup>
+    <PackageReference Include="FSharp.Core" Version="4.7.1" />
     <PackageReference Include="JetBrains.Annotations" Version="2019.1.3" />
     <PackageReference Include="Newtonsoft.Json" Version="12.0.3" />
     <PackageReference Include="squirrel.windows" Version="1.5.2" />

Plugins/HelloWorldFSharp/HelloWorldFSharp.fsproj

@@ -29,4 +29,8 @@
     <ProjectReference Include="..\..\Flow.Launcher.Plugin\Flow.Launcher.Plugin.csproj" />
   </ItemGroup>
 
+  <ItemGroup>
+    <PackageReference Update="FSharp.Core" Version="4.7.1" />
+  </ItemGroup>
+
 </Project>