Add pendingRequestCount field to BasicCrawler

[jancurn]

This will be similar to handledRequestsCount, but it will indicate how many requests are yet to be processed. The users of BasicCrawler can then use this field to determine whether they need to enqueue more requests during recursive crawling (there's no point to do it if you know there are too many already).

[mnmkng]

Related: https://github.com/apifytech/apify-js/issues/321