Depend on R 3.6 + other package cleanup

[olivroy]

Ready for review! I don't think I will have anything more to add relates to #1626

Summary

Depend on R 3.6 to be able to use features of R and remove workarounds. Not sure if gt fully worked on older versions. isTRUE() is already used in certain places for example. (Note isTRUE() existed before isFALSE()

• allows for removal of tidy_sub() and tidy_gsub() (for consistency in the codebase, it is possible to simply use gsub() and sub() in most cases.
• Tested that the tidy_sub(), tidy_gsub() test passed
• Remove is_false() (isFALSE() introduced in R 3.5.)
• Allows for removal of register_s3_method(), see https://www.tidyverse.org/blog/2024/01/roxygen2-7-3-0/ for why/ (fiddle a bit with as.tags() in 8b7f9d8

Unrelated

• Don't ignore snapshots that are not ignored on CRAN in 64d0bb2 (to actually test that output is consistent across versions)
• Move tidy_grepl() to tests/testthat/helper.R since unused in code. https://testthat.r-lib.org/articles/special-files.html
• Change gsub() to trimws() for clarity dd5a471
• Change gsub() begin end to paste0() for clarity e93c5db.
• Comment about names not NULL. Maybe the condition is incorrect, and this may relate to Setting cols_width causes column to "disappear" #1510 (maybe not)
• Change of option. 18c10b5 (I just find it clearer this way, let me know if you disagree)
• Refactoring in 78e6a29 (caused previous build failures, resolved with 90337a0)
• Rename function for clarity in 02c97e3 to allow removing comments everywhere, and synthetize it in the helper function instead (since the comment is unrelated to individual functions)
• Simplify some dplyr code in 8da30ca
• Was able to remove a workaround introduced in silence tests on Windows #1634 due to edbd9f4
• Remove Collate and move imports to gt-package.R to avoid repetition

Checklist

• I understand and agree to the Code of Conduct.
• I have listed any major changes in the NEWS.
• I have added testthat unit tests to tests/testthat for any new functionality.

FYI, the hard-to-debug failure was due to r-lib/testthat#1646

[rich-iannone]

Thanks for all the great work! This is a nice round of cleanups and I agree with everything done here. Will review and merge shortly.

[rich-iannone]

LGTM!