Support `list.files()`/`dir()` in `fixed_regex_linter()`

[Bisaloo]

Currently in R-devel (R 4.6.0):

list.files() now has optional argument fixed to allow literal pattern matching. Suggested and implemented by Duncan Murdoch in PR#18925.

[MichaelChirico]

Nice! I typically like to wait for a released version with the feature before working on it (it might be reverted, or delayed to a future release)