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).
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