lib.strings.trimWith: Refactor scoping (#339001)

      start ? false,

      end ? false,

    }:

    s:

    let

      # Define our own whitespace character class instead of using

      # `[:space:]`, which is not well-defined.

          "(.*[^${chars}])[${chars}]*"

        else

          "(.*)";

    in

    s:

    let

      # If the string was empty or entirely whitespace,

      # then the regex may not match and `res` will be `null`.

      res = match regex s;