Implement unit parameter in st_make_grid_ext() building on v. 1.0-11 updates to sf::st_make_grid()

[elipousson]

Version 1.0-11 of sf adds support for area units to the cellsize parameter for sf::st_make_grid(): https://r-spatial.github.io/sf/news/index.html#version-10-11

I'd like to add support for a unit parameter to st_make_grid_ext() so it works similarly to st_buffer_ext().