Add *.props as a recognized file extension for XML

[AArnott]

Several MSBuild file extensions where already there including *.targets.

*.props is becoming increasingly important especially with .NET SDK projects using Directory.Build.props so frequently.

[mention-bot]

@AArnott, thanks for your PR! By analyzing the history of the files in this pull request, we identified @Tyriar and @egamma to be potential reviewers.

[msftclas]

@AArnott,
Thanks for your contribution as a Microsoft full-time employee or intern. You do not need to sign a CLA.
Thanks,
Microsoft Pull Request Bot

[aeschli]

Thanks @AArnott !