goldmark-jupyter: support cell attachments in markdown cells

[bevzzz]

This PR adds a package with extensions for goldmark and nb that, together, add support cell attachments in markdown cells.

Some important remarks:

• While nb aspires to work with different markdown renderers, not all of them have an extension API that's permissive enough to allow for a similar implementation. This package may serve as a POC / template for extensions that target other renderers.
• Not publishing goldmark-jupyter as a standalone module (in a separate repo), because it is rather niche and wouldn't be of much use outside of nb "ecosystem".

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b138fc5) 82.13% compared to head (d694010) 82.13%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main       #5   +/-   ##
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  Coverage   82.13%   82.13%           
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  Files          11       11           
  Lines         571      571           
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  Hits          469      469           
  Misses         81       81           
  Partials       21       21           

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