collapse [markdown] cells in .py files

[YoniChechik]

I guess that by enabling this feature, the clutter in the file could be smaller

[rchiodo]

I don't believe this is possible using VS code's API. It seems like it should be (as they provide collapsible ranges for code) but not sure we could do this without overwriting the typescript grammar.

[greazer]

Dupe of #1527