string_boundary_linter

[MichaelChirico]

This is a linter that looks for usages that should be startsWith() or endsWith() that use e.g. grepl("^fixed_string", x), substring(x, 1, 4) == "abcd", etc.

Pushing to add it to the 3.0.0 release so that our internal package doesn't need to keep maintaining the C code associated with fixed_regex_linter now that it's R-based in lintr.