add range version of setExtension

Since Walter is being unresponsive, I'm hijacking his [original PR](https://github.com/D-Programming-Language/phobos/pull/2149). This PR basically uses Walter's code as base, and adds some fixes on top per reviewers' comments.
Using chain will auto-decode which makes it throwable and may gc allocate. Use `path.byCodePoint`, ditto for `ext`.