Suggested feature: code folding

[danmyles]

A useful enhancement would be to enable code_folding in tufte_html. This issue has already been raised on stack overflow, but has as yet received no response: https://stackoverflow.com/questions/64428978/rmd-and-folded-code-for-tufte-html-output

Something like this in the header would be +++, as it aligns with implementation using other themes.

title: "Code folding is great"
author: "Dan Myles"
output:
  tufte::tufte_html:
    code_folding: hide

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