Rather than use our own custom expect library, we can use expect from npm [1], which has an API almost identical to the one Puppeteer has, but with more options, better diffing, and is used by many in the community as it's the default assertions library that comes with Jest.
It's also thoroughly documented [2].
[1]: https://www.npmjs.com/package/expect
[2]: https://jestjs.io/docs/en/expect
This updates our `tsconfig.json` so it emits our JavaScript files as
well as type checking them. We compile into `./lib` which we then ship
in our npm package. The source code has moved from `./lib` into `./src`.
Because the `src/` directory is exclusively JS files, this change is a
no-op in terms of code functionality but is the first step towards being
able to replace `src/X.js` with `src/X.ts` in a way that allows us to
migrate incrementally.
The `lib` directory is gitignored, and the `src` directory is
npmignored. On `npm publish` we will now run `npm run tsc` in order to
generate the outputted code.
