* chore: move `index.js` into `src`
This is the first part of a series of pull requests that will slowly
make it possible to initialise Puppeteer for either a Node environment
or a web one. By creating the `initialisePuppeteer` function we can
inject dependencies in for the given version of Puppeteer (e.g. inject a
different debug library for Node vs the web).
This PR moves the initialisation into `src` and calls into it from the
root `index.js`.
