refactor: Switch to Vitest from Jest

[cephalization]

Benefits:

• Speed
• Single build pipeline instead of two
  • Now app build and test runs both use the same vite config instead of one vite build and one esbuild build
  • Build plugins now work for builds and tests, no longer need to duplicate
• Interesting future features
  • "Browser Mode" testing that can run your unit tests in playwright instead of node + jsdom, meaning unit tests will catch browser runtime issues that won't be caught in node

[cephalization]

More selfishly, this enables tests to be written for #4943, which currently adds a rollup plugin incompatible with jest/esbuild