Feature request: Collapsible markdown headings

[akshayka]

A feature, in both edit and run mode, to collapse cells by markdown heading level; similar to what Colab has.

Requested by a user on discord.

Original request: i think it does help navigability of the notebook. also, like for jupyter notebooks on colab, one of the main purposes (in comp bio at least) is colab notebooks as a plug and play way of operating on your own data (e.g. alphafold has a colab notebook you can run with). those kinds of notebooks get long though so having collabsible sections really helps there. given that marimo is especially interested in that kind of space it seems with sharing interactible notebooks as apps i think that would def be helpful