feat: refactor writing to buffer

[Rasmus-Bertell]

Makes writing to the buffer cleaner and easier to add more blocks.
This is pre work for #256 so that it's easier to add the statistics block into the output.

This shouldn't affect the output other than it removes the line #+END between the command and headers on subsequent requests if the buffer wasn't closed. It was leftover since the buffer wasn't cleared properly.

[NTBBloodbath]

This is awesome, thank you so much. The codebase is much more readable now for my eyes and I hope it is for others too. Also fixed a small bug along the way so excellent :)