Test existence of EDITOR before attempting to use it

[MichaelChirico]

Ran into this ugly output from usethis::use_package_doc() on a GitHub codespace (which is intentionally lightweight):

✔ Setting active project to "/workspaces/lori".
✔ Writing R/lori-package.R.
☐ Modify R/lori-package.R.
sh: 1: vi: not found
☐ Run `devtools::document()` to update package-level documentation.
Warning message:
error in running command

The problem is R prefers EDITOR=vi by default (even if it's not available).

A more advanced fix might instead come up with a {usethis}-internal ordered list of editors to try, iterating over them and using the first found installed. But anyway this PR is a good first improvement.

I started an r-devel thread too:

https://stat.ethz.ch/pipermail/r-devel/2024-December/083717.html