Provide pattern synonyms for the AST?

I found it useful to have pattern synonyms like these: https://github.com/aelve/cmark-patterns/blob/master/lib/CMark/Patterns.hs. Perhaps they could be added to `cmark` itself? (If not, I'm happy to keep them in my library.)

Review lib/CMark/Patterns.hs in the linked cmark-patterns project and the cmark-hs AST bindings first. Compare the existing pattern set with the public AST API, then determine which patterns belong in cmark itself. Done means the agreed pattern synonyms are exposed without breaking the existing interface.

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