chore: move code to src/ and emit with TypeScript (#5568)

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.

.eslintignore

@@ -7,3 +7,4 @@ node6/*
 node6-test/*
 node6-testrunner/*
 experimental/
+lib/