create SourceLink.Embed.AllSourceFiles nupkg

[ctaggart]

An alternative to embedding source link json, is just to embed all the sources. It is more difficult right now that it should be. It also is different in C# than in F# projects. It would be great to allow users to switch easily between.

dotnet/roslyn#19127

[ctaggart]

Like SourceLink.Create there will be at least a couple variations of embed. I'm thinking:

• SourceLink.Embed.AllSourceFiles
• SourceLink.Embed.PaketFiles

Both should simply be this, but the first one without the condition.

https://github.com/fsprojects/Paket/blob/master/src/Directory.Build.targets

<Project>
  <PropertyGroup>
    <BuildDependsOn>EmbedPaketFiles;$(BuildDependsOn)</BuildDependsOn>
  </PropertyGroup>
  <Target Name="EmbedPaketFiles">
    <ItemGroup>
      <EmbeddedFiles Include="$([System.IO.Path]::GetFullPath('%(Compile.Identity)'))"
        Condition="$([System.String]::Copy('%(Identity)').Contains('paket-files'))" />
    </ItemGroup>
    <CreateProperty Value="@(EmbeddedFiles)">  
      <Output TaskParameter="Value" PropertyName="embed" />  
    </CreateProperty>
  </Target>
</Project>