New lint equivalency fuzzer: different forms of quoting (' vs " vs R"()")

> Another potential fuzzer: modify string formats. Switching `'` to `"` and try raw strings. > > Linters that correctly use `get_r_string()` should pass fine but maybe we are missing cases where it should be used. _Originally posted by @Bisaloo in [#2191](https://github.com/r-lib/lintr/issues/2191#issuecomment-3125892009)_