Add support to the existing marked markdown preset for footnotes

[lschierer]

it turned out to be trivial to add GFM footnote support with marked-footnote to the existing marked markdown preset. I suppose this could be optional, but I'm not sure how to add in an option to the preset plugin's config, though if such an option existed, it would be easy to change the way marked is called based on that option. As it stands, with this patch, it is on all the time.

[JulianCataldo]

Hey! Thanks for the idea.

I think it is a reasonable goal for the minimal preset to be able to support GFM entirely.
Tables etc. are already extended syntax / GFM-flavored, supersets of CommonMark.

Footnotes are expected to be supported IMO. They're kind of standard and widespread. I personally don't use them or encounter them very much though.

I've thoroughly checked and the dep's graph is shallow (just one package), so it will not make the preset too fat, which would defeat the purpose.

I'm gonna test it locally and merge it ASAP.

I think GitHub admonitions could be a pretty good addition, to: https://github.com/bent10/marked-extensions/tree/main/packages/alert
Those I use extensively.
I expect READMEs in a GH repo to be displayed with the same features if ingested into a website renderer, no more, no less.