Create an analyzer

Been thinking about doing this for a couple of years now. Some things that would be good to have:

- Classes implementing ITestBlock but that don't have an Execute() method.
- TestBlocks with an async Task signature but are added via AddTestBlock
- Using .Build().Execute() instead of .Build().ExecuteAsync()

Start by locating ITestBlock, Execute, ExecuteAsync, AddTestBlock, and Build in the repository, then review how tests and diagnostics are organized. Define analyzer coverage for the three listed patterns and verify that each is reported with focused tests; done means all requested misuse cases are detected without flagging valid usage.

Written by the indexing model from the issue text.