Crawl using Node.js fetch function directly

[tidoust]

Through fetch-filecache-for-crawling, Reffy used to depend on node-fetch to send HTTP requests. The fetch-filecache-for-crawling library now uses the native implementation of fetch in Node.js v18 and above. This update bumps the version of fetch-filecache-for-crawling, making Reffy use the native implementation of fetch in Node.js instead.

This update also removes the dependency on the AbortController polyfill, since it is no longer needed.

On top of a couple of changes needed to account for slight differences in the way headers are handled, main changes in this update are test-related, to replace the nock library, which can no longer be used because it cannot intercept fetch requests, with the mock functions of undici (which provides the implementation of fetch in Node.js).

From a crawling perspective, this update is a no-op, although it should be noted that, when certain specs are crawled (MathML Core in practice), Node.js may report memory leak warnings such as:

MaxListenersExceededWarning: Possible EventTarget memory leak detected. 101
abort listeners added to [AbortSignal]. Use events.setMaxListeners() to increase
limit

These are due to the fact that the same AbortController is (rightly) connected to all pending HTTP requests linked to the spec being crawled, and the MathML Core draft references over 100 embedded resources. In other words, that's all normal!

The AbortSignal implementation in Node.js does not directly inherit from EventEmitter. As far as I can tell, there is no direct way to call setMaxListeners() as a result.

If that proves annoying over time, we could rather adjust events.defaultMaxListeners but that is a global setting: https://nodejs.org/dist/latest-v18.x/docs/api/events.html#eventsdefaultmaxlisteners

(Alternatively, it may be worth digging into MathML Core to check whether we actually need to download all of these resources).