This PR adds a `PathExt` trait. It pulls in our existing `compact()`
function, as a method, and then adds a method, and testing, for
`icon_suffix()`. A test was added to fix:

- https://github.com/zed-industries/community/issues/1877

Release Notes:

- Fixed a bug where file icons would not be registered for files with
with `.` characters in their name
([#1877](https://github.com/zed-industries/community/issues/1877)).