Add intellisense xml's to ref-pack

[vatsan-madhavan]

This issue tracks the inclusion of intellisense xmls' in our ref-assembly nuget package.

We still need to define the following:

• Who produces the intellisense xml's
• How do we consume them (checked into our repo vs. consumed as a nuget package)
• Where do we put them (presumptively, into the reference assembly packages, Microsoft.DotNet.Wpf.GitHub, and Microsoft.DotNet.Wpf.DncEng. => These will get consumed by core-sdk, unpacked and repacked into the core-sdk re-packs.

Tracked by internal work item https://devdiv.visualstudio.com/DevDiv/_workitems/edit/896312