More informative names for gradient scales

[teunbrand]

This issue separates out a discussion about (re)naming gradient scales from #3738.

To briefly recap, having sequential or diverging continuous scales is conceptually an important difference. The current naming of gradient scales does no favours distinguishing these types of scales. It was proposed herein that scale_{fill/colour}_gradient2() be renamed to scale_{fill/colour}_gradient_div().

I'd like to add to this proposal that scale_{fill/colour}_gradient() itself be renamed to scale_{fill/colour}_gradient_seq() to set these apart. scale_{fill/colour}_gradientn() can be used for either purpose, so I'll suggest that we leave that name as-is.