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.

索引モデルが issue の本文から書いたものです。