Refactor markdown event collection

I don't even remember how I did that, except that it was a weird combination of `peek`, `take_while` and macros. We can do that in a better, maintainable way. And maybe even find a nice abstraction for common things.

Also, itertools is our friend. And we shouldn't worry about performance.

The issue does not name a file or test; start by locating the markdown event collection that uses `peek`, `take_while`, and macros. Read the surrounding collection code and existing abstractions, then use the project's tests or checks to confirm the refactor preserves behavior while making the implementation more maintainable.

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