Add an option to convert all potential String.Format() calls to string interpolation

[DustinCampbell]

[Bryan-Legend]

As for providing a fix all option, that'll take a bit more design. This is implemented as a refactoring rather than a diagnostic/code fix pair, so the "FixAllProvider" APIs aren't available. I chose to implement this as a refactoring because this operation struck me as something that you might not always want to do. So, it seemed a bit invasive to squiggle every potential String.Format() call that could be converted to string interpolation.

You are absolutely right, there should never be squiggles on all the String.Format calls. But I would still like the the convert all options to be available when I want them. Same with "Use Expression-Bodied Member", I'd like convert all available but hate all the messages. The FixAll options should be available independent of the warning level. Seems like a change to VS is needed.