Remove Regex from analyzers

[MattKotsenas]

Replace using a Regex on string-ified type names with the proper Roslyn API (usually Identifier.Name) and do some 'straightening' of the code using early-returns and pattern matching to reduce the triangle-ness.

This is effectively a rewrite of AsShouldBeUsedOnlyForInterfaceAnalyzer. There's a smaller refactor for CallbackSignatureShouldMatchMockedMethod* as that code needs to be shared between the analyzer and the code fix provider (will be handled later)

This refactor has a few benefits:

1. It addresses the bug mentioned in Upgrade (or at least test) with a newer version of Moq #58 (testing with a newer version is coming in another PR to keep code reviewable)
2. It improves performance a good amount using very basic Visual Studio tracing (e.g. ~20 fewer string allocations and saves 30% [20 ms] on a single call to Analyzer.Analyze())
3. It plumbs the CancellationToken into more places to be a good IDE citizen

This PR also starts to break apart the monolithic Helpers class into smaller components. In order to support the .TryXXX pattern I needed to bring in a polyfill for [NotNullWhen] for nullability analysis. I'm using PolySharp because, in addition to being the most popular polyfill, it uses a source generator to add polyfills to the current assembly, which avoid assembly loading overhead and the hassle of adding dependencies to the analyzer.