Add performance benchmarks

[peddybeats]

Deliverables:

1. A script that brings up the app, runs for N seconds and averages the # of inferences per second
2. Configuration that runs this as a part of CI to ensure changes don't cause performance regressions
3. A table in README that list inferences per second on {macbook pro, pixel 3, iphone 11, thinkpad} x {wasm, webgl, cpu}

Note, in CI, the webcam feed may need to be mocked using pre-recorded footage; if it proves to be too challenging, we may have to split 2 into an issue for later.

[Berkmann18]

Should we have Lighthouse as part of the CI? That way, metrics like performance and a11y can be automatically tested.

[peddybeats]

I've never used Lighthouse, but just did a quick search and seems like an excellent fit!
https://github.com/treosh/lighthouse-ci-action

Also just noting in case it's useful, this is the page tfjs uses to do benchmarks:
https://github.com/tensorflow/tfjs/blob/master/tfjs-core/benchmarks/index.html