[lecture enhancement] code cell collapse/fold

[Shu Hu]

Hi @mmcky and @jstac , I find that we have a very good feature to collapse/fold a long-line code cell in our RST-version lectures.

Do you think it is a good idea to translate this feature to the new MYST lectures as well?

e.g., in lecture amss (left: RST; right: MYST):

[Matt McKay]

@shlff thanks.

This is now supported in the theme using collapse-20 tag for code-cells

QuantEcon/quantecon-book-theme#128

Would you mind raising pull request to update each lecture where the collapse was previously used?

[Matt McKay]

@AakashGfude for collapse can you just specify collapse for the default 20 lines or do you have to use collapse-*.

@shlff let's use collapse-20 to be explicit.