refactor(skipLast): rename skipLast's parameter to count

[jakovljevic-mladen]

I renamed skipLast's parameter from skipCount to count for two reasons:

• the docs in the first sentence is still referring to this parameter named as count;
• to have to same name as the other operators (e.g.skip, take or takeLast).

[benlesh]

I'm not sure this makes the code any better. If anything, it's less readable now with count, counter and currentCount. Admittedly it wasn't great before, but at least it was obvious which one came from the argument.

[benlesh]

@jakovljevic-mladen I've added a related PR, and I'd be happy if you reviewed it: #5788