* chore(agnostify): Create Node and Web initializer.
This PR splits `initialize.ts` into two files, one for web, and one for
Node. The Node initializer requires much more information as it needs to
know which browser(s) to download and where to store them, whereas the
web one does not.
A future PR that I'm working on will tidy up `src/common/Puppeteer.ts`
(as it contains a lot of Node specific logic around downloading and
installing browsers), but this change enables us to stop the browser
bundle attempting to use the `pkg-dir` dependency, which wouldn't work
within a browser, as well as keeping the size of the PRs down and
avoiding one mammoth PR.
