The `Puppeteer` class had two concerns:
* connect to an existing browser
* launch a new browser
The first of those concerns is needed in all environments, but the
second is only needed in Node.
https://github.com/puppeteer/puppeteer/pull/6484 landing enabled us to
pull the `Puppeteer` class apart into two:
1. `Puppeteer` which hosts the behaviour for connecting to existing
browsers.
2. `PuppeteerNode`, which extends `Puppeteer` and also adds the ability
to launch a new browser.
This is a non-breaking change, because Node users will still get an
instance of a class with all the methods they expect, but it'll be a
`PuppeteerNode` rather than `Puppeteer`. I don't expect this to cause
people any issues.
We also now have new files that are effectively the entry points for
Puppeteer:
* `node.ts`: the main entry point for Puppeteer on Node.
* `web.ts`: the main entry point for Puppeteer on the web.
* `node-puppeteer-core.ts`: for those using puppeteer-core (which only
exists in Node, not on the web).
These files will be used by both the web and node versions of Puppeteer.
Another name for this might be "core" but I don't want to cause
confusion with the puppeteer-core package that we publish at the moment.
