refactor(test): moving unit testing over to Vitest

[erunion]

🧰 Changes

I've been extremely impressed with Vitest!

Before

npx nyc mocha "test/**/*.test.ts"  4.20s user 0.34s system 35% cpu 12.645 total

After

 Test Files  5 passed | 1 skipped (6)
      Tests  31 passed | 8 skipped (39)
   Start at  19:42:15
   Duration  5.42s (transform 230ms, setup 0ms, collect 653ms, tests 9.71s, environment 1ms, prepare 313ms)

npx vitest run  3.19s user 0.73s system 66% cpu 5.885 total

We unfortunately have to scuttle our browser tests for the moment though because Vitest's browser mode is still very experimental. When that's a bit more stable, and doesn't crash upon trying to run tests in a browser, I'll re-enable those in here.