* The testing tsconfig.json inherits from the base TS config.
* A lot of type assertions have been inserted...a lot.
* All testing utilities have migrated to TS.
* text-diff is being replaced with diff for TS compatibility.
* ProtocolError has been added to PuppeteerErrors and PuppeteerErrors is no longer a record (it's been frozen).
* Fixes a small bug where null was an allowable media type in emulation (should be undefined).
The `Launcher` class was serving two purposes:
1. Launch browsers
2. Connect to browsers
Number 1) only needs to be done in Node land, but 2) is agnostic; in a
browser version of Puppeteer we'll need the ability to connect over a
websocket to send commands back and forth.
As part of the agnostification work we needed to split the `Launcher` up
so that the connection part can be made agnostic. Additionally, I
removed dependencies on `https`, `http` and `URL` from Node, instead
leaning on fetch (via `node-fetch` if in Node land) and the browser
`URL` API (which was added to Node in Node 10).
