feat: add vitepress

[TheAlexLichter]

Summary

This PR adds VitePress detection.

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[serhalp]

Thanks, Alex! LGTM. Could you please just take a stab at adding a test? It should be straightforward to copy a similar one like this. I'm partial to pasting the scripts and deps from a real vitepress site into the fixture if you can. 🙏🏼

[TheAlexLichter]

Will do ☺️