Add `pixel_areas` helper

[dcherian]

I've wanted this before --- get a DataArray of pixel areas given a Dataset --- but couldn't figure out how to do it generically.

cc @scottyhq

[dcherian]

One reference: https://gis.stackexchange.com/questions/127165/more-accurate-way-to-calculate-area-of-rasters

[mdsumner]

Look at terra::cell size https://rspatial.github.io/terra/reference/cellSize.html

This is crs-independent, unless "transform=FALSE". The core is written in C++ in terra's internal Spat lib. I'd be keen to help guide this.

[mdsumner]

Btw anything by whuber or cffk is absolutely authoritative so that SO post is definitely good. Maybe it's implemented in Karney's GeographicLib (it's an excellently simple library to use)