New grepv_linter() for more concise string subsetting

[MichaelChirico]

Potentially in a follow up PR, but if we start recommending grepv() (which I agree with), it would be good to also lint grep(, value = TRUE).

Yea, I've been meaning to file that as an issue for a new linter (presumably grepv_linter(), I'm not sure if maybe it should be subsumed in another linter, probably not).

Originally posted by @MichaelChirico in #2856 (comment)