Analyzer to encourage canceling work on shutdown

[AArnott]

Problem statement

Too much code in Visual Studio runs expensive, async work that slows down IDE shutdown.

Proposed solution

A new analyzer that ensures all JoinableTaskFactory.SwitchToMainThreadAsync calls pass in a CancellationToken (or None or default), with a fix offered to pass in VsShellUtilities.ShutdownToken. The analyzer should only activate when the VsShellUtilities.ShutdownToken symbol is available to the compilation.