Transition to node's fetch

[coolaj86]

Node is getting fetch as a default global, but it's not there yet:

(node:7796) ExperimentalWarning: The Fetch API is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)

When Node.js LTS stops issuing that warning we may be able to switch to the webpack-ready browser.js (forthcoming) to being the main for both.

The API would be kept largely the same, but at the very least there will be subtleties that require a breaking change major version bump, so perhaps we'll drop a few old features too.

Why not abandon request in favor of fetch?

In many cases that should be done, but there are a few things - such as handling opts.auth- that require a small amount of utility code.

Node's fetch vs WHATWG fetch

See https://github.com/nodejs/undici/#specification-compliance.

In summary:

• you must consume or cancel the body
• you can get and set forbidden headers, such as Set-Cookie, Cookie and User-Agent
• (duh) Node buffers and Node streams are allowed
• caching is (probably?) not supported