roslyn: Analyzer to enforce "Async" suffix on methods returning tasks

It’s quite common that I call out the need for an Async suffix on a method name during code review.

Given this is a standard convention, would a diagnostic that flagged violations make sense?

It would improve productivity by reducing feedback loops in several projects for which I review PRs.

Another convention such an analyzer may wish to support is When* methods, such as Task.WhenAll and friends, which do not end with Async.