string_boundary_linter() should recognize substr(x, 0, 1) as well

As observed on CRAN: https://github.com/search?q=org%3Acran+%2Fsubstr%5C%28.*0%2C%5Cs*1%5C%29%2F+lang%3AR&type=code ``` substr('abc', 0, 1) == substr('abc', 1, 1) # [1] TRUE ```