Add 'scan for unreferenced files' tool

[DrusTheAxe]

You can easily create a file and not add it to a VS project, or Add a New file via Visual Studio but then Remove it from the project but not delete it. This unused file won't be visible in Visual Studio's Solution Explorer but is to other tools that look at the file system (grep, programmer's editors, etc). This problem was noticed recently.

It'd be helpful to have a ScanForUnreferencedFiles tool that would crawl the repository and list all such 'unused' files that shouldn't exist. Developers should be able to manually run this scan locally. PRs can run it too, though we may want to consider its findings non-blocking-warnings and not blocking-errors. It may also be good to run recurringly (monthly?) as part of the build process, or release checklist process, etc to identify 'dead' files we should delete and not accidentally keep around.