Warning about large number of cells only comes after cells are created

[mpjashby]

Describe the bug
hotspot_grid() produces a warning if the created grid contains more than a certain number of cells, because this makes hotspot_kde() etc. very slow. But this warning is only triggered after the grid itself is created, which means that if creating the grid itself takes a long time then the user is left with a function that takes a long time to run and provides no feedback.

Expected behavior
The warning should be triggered before the grid is calculated, based on calculating approximately how many cells the grid will have once it's created. The warning should also be extended so that it provides some tips, e.g. If you are trying to calculate a grid for a very large area, consider using a larger `cell_size` or clip your data to a smaller area using `st_intersection()` .

[mpjashby]

By default, warnings are only output after the function in which they're created has returned. In base R there is a way around this, but I can't find a way around it in rlang. I've created a question on Stack Overflow to see if there is a workaround or if I've missed something: https://stackoverflow.com/q/77123990/8222654

[mpjashby]

It seems that immediate warnings aren't possible in rlang and aren't going to become possible: r-lib/rlang#1546

The least-worst workaround seems to be to use rlang::inform() instead, since messages are outputted immediately: https://stackoverflow.com/a/77142044/8222654