Fsharp: enable SourceLink for FSharp.Core.pdb

[ctaggart]

Following up on #294, please enable SourceLink for the published FSharp.Core.pdb files so that we can debug into the source code just like the rest of dotnet core.

[7sharp9]

We could do with this asap, stepping through F# code as C# is not fun in the debugger. Im sure people contributing to the compiler and tools find this painful too as you have to read C# code and cross reference it with the repo here to get a clear picture of what happening.

[baronfel]

This _should_ just be a matter of adding a dependency to Microsoft.SourceLink.Github, version 1.0.0-beta2-19367-01 and setting a few additional properties in the FSharp.Core Project File (or maybe the FSharpBuild.Directory.Props, depending on conventions?):

    <!-- Optional: Publish the repository URL in the built .nupkg (in the NuSpec <Repository> element) -->
    <PublishRepositoryUrl>true</PublishRepositoryUrl>

    <!-- Optional: Embed source files that are not tracked by the source control manager in the PDB -->
    <EmbedUntrackedSources>true</EmbedUntrackedSources>

    <!-- Optional: Build symbol package (.snupkg) to distribute the PDB containing Source Link -->
    <IncludeSymbols>true</IncludeSymbols>
    <SymbolPackageFormat>snupkg</SymbolPackageFormat>

[cartermp]

I think https://github.com/dotnet/fsharp/pull/9948 should resolve this issue.