feat: Reinstate lint infrastructure as GHA

[MichaelChirico]

This started from noticing test-lint.R is not really doing anything, and that {lintr} should probably be removed as a package dependency.

All changes to package code here are suggestions, I'm happy to revert anything.

Most of the changes are from the expect* series of linters that recommend "best practices" for {testthat} usage.

[krlmlr]

Thanks, this looks much cleaner overall.

R CMD check is unhappy: "Undefined global functions or variables: NUL"

What automation (if any) do you use to remove lints everywhere?

It will be easiest for me to merge a slew of the changes up front, this might take some time.

[MichaelChirico]

What automation (if any) do you use to remove lints everywhere?

Just regex replacement and multicursor mode

It will be easiest for me to merge a slew of the changes up front, this might take some time.

OK, take your time. Also happy to split it up if that makes more sense.

[MichaelChirico]

@krlmlr is {constructive} generating these? maybe I need a devel version?

[krlmlr]

Thanks. I've tweaked. Not a fan of nzchar(), and one of the toString() refactorings looked strange to me. Can you please make lintr happy? Other than that, ready to merge.

[krlmlr]

Thanks! Need to proceed, will clean up remaining lints as needed.