fix: handle root in join and directory_name

[JackoCoolio]

Fixes #9.

Previously, joining an empty string with an absolute path (and vice-versa) would result in a relative path. Now, it returns the absolute path.

As a side-effect, directory_name("/") returns "/", where it used to return an empty string. This matches the behavior of C's dirname("/") and Erlang's filename:dirname("/").

[lpil]

Lovely. Could you update the changelog please 🙏

[JackoCoolio]

Added a "v1.1.3" section to the changelog.

[JackoCoolio]

Hmmm, I initially wrote this to address lustre-labs/dev-tools#173 but I'm realizing now that this alone isn't enough. We'd need join("some/path", "/an/absolute/path") to return "/an/absolute/path".

For reference, that's how Erlang's filename:join/2 and Python's os.path.join work, but NodeJS's require("node:path").join returns "some/path/an/absolute/path". It has a separate resolve function for the alternate behavior.

Do you think we should update join to return "/an/absolute/path" here, or add a new resolve-like function? Of course we could just leave it as is and let clients figure it out lol