Avoid testthat deprecated features + upkeep

[olivroy]

Basically, this updates the Roxygen2 version and use "_PACKAGE" https://www.tidyverse.org/blog/2024/01/roxygen2-7-3-0/

• Remove crayon from Suggests
• Update GitHub actions with usethis::use_github_action() (remove pkgdown action, not needed due to Ropensci publishing site)
• Use testthat 3rd edition
• Remove testthat::with_mock() as it is now deprecated and replace it with testthat::local_mocked_bindings()
• The larger change is to transition to snapshotting to verify output. I isolated that in separate commits. Basically, the idea is to use expect_snapshot(), and for testing knit_print(), I used cat() to only capture output.

I verified manually that the new output is the same as the old.

As you see, I didn't modify much under R/.

Happy to answer questions

• https://testthat.r-lib.org/articles/third-edition.html
• https://testthat.r-lib.org/articles/snapshotting.html