chore: upgrading vitest to v3, laying ground work for vitest browser …

…testing (#1054)

## 🧰 Changes

This upgrades us to `vitest@3` and also lays some initial ground work
for eventually moving our Jest browser tests into Vitest (so we aren't
running two testing frameworks).


[demo]: https://markdown-pr-PR_NUMBER.herokuapp.com
[prod]: https://SUBDOMAIN.readme.io
[icn]:
https://user-images.githubusercontent.com/886627/160426047-1bee9488-305a-4145-bb2b-09d8b757d38a.svg