chore: migrate tests to Vitest

[manzt]

This is an experiment to replace tape-* with Vitest. We are already using Vite for development, and Vitest works really nicely and can mock many different environments.

Writing tests with Vitest are much more ergonomic, and we can avoid tests with many assertions in favor of separate tests with a single assertion with teh test.each usages.

So far I've only ported the tests in loaders, so to try this out you'll need to:

pnpm i
cd packages/loaders
pnpm vitest