📦 Vendor markdown-it-myst-extras into markdown-it-myst

[agoose77]

We spoke a while ago about doing this in the context of colon-fence handling. Presently, the markdown-it-myst-extras package lives under the executablebooks repo. There are still other plugins that live there, but I think for now that's OK — we might want to build them in our monorepo going forward.

I decided to deprecate the default export — it feels unusual to only enable half of the plugins. Feel free to push back there.

c.f. #2773

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[rowanc1]

This looks good to go from code review. @agoose77 if you are confident this is faithfully pulling those tools in, lets merge!