Add support for Visual Studio Team Explorer 2017 by jcansdale · Pull Request #961 · github/VisualStudio

From the Visual Studio website:

"Visual Studio Team Explorer 2017 is a rich, standalone client for accessing Team Foundation Server and Visual Studio Team Services and is free for any user. This install only includes Team Explorer so you can access version control and work item tracking without other IDE components."
https://www.visualstudio.com/en-us/news/releasenotes/vs2017-relnotes#te

At the very least, adding support for this could be useful for testing (a minimal install of VS with just the packages we need). It's also an obvious target for our extension!

At the moment all this PR does is allows the extension to load when the InstallationTarget is "Microsoft.VisualStudio.IntegratedShell".

We will likely want to check what components are available when loading the extension so we don't fail in any IntegratedShells that don't have Team Explorer available.

Fixes #938