Add a ponyfill export

[zacharyliu]

Currently, the two ways of consuming this package have downsides:

1. The abort-controller export relies on the native implementation in browsers, which will not work in older browsers
2. The abort-controller/polyfill export patches the global AbortController if not present, which can be undesirable

It would be handy to support a ponyfill export which can offer the benefits of both.

(It's likely that this package is used in combination with cross-fetch, which encourages the use of its ponyfill export in its README.)