shift to `sf`

[ernestguevarra]

Points to consider:

• Error in st_sample() with type = "hexagonal" for unprojected polygons r-spatial/sf#1220 (comment)
• st_sample with regular sample r-spatial/sf#771
• https://r-spatial.github.io/sf/reference/st_make_grid.html

[ernestguevarra]

maybe best to just create a separate set of functions that uses sf rather than replacing? and then over time, can decide whether to deprecate sp-based functions.