This is another step towards making Puppeteer agnostic of environment
and being able to run in Node or a browser.
The files in the `node` directory are ones that would only be needed in
the Node build - e.g. the code that downloads and launches a local
browser instance.
The long term vision here is to have three folders:
* node - Node only code
* web - Web only code
* common - code that is shared
But rather than do that in one PR I'm going to split it up to make it
easier to review and deal with.
* chore: extract `BrowserRunner` into its own module
`src/Launcher.ts` is large and hard to work in. It has multiple objects
defined in it:
* ChromeLauncher
* FirefoxLauncher
* BrowserRunner
* Launcher
This change moves BrowserRunner into its own module. More refactorings
like this will follow but this is the first step.
* chore: remove src/externs.d.ts
It defined global types that we don't want to use, and instead we move
to using interfaces that we import and reference just like with any
other interface.
This means other than Protocol (which I think is fine to leave as is),
there are no other magic global types and you have to import any types
or interfaces that you want.
