The name `path-extension` created a conflict for an existing
registered package, so it should not have been added to
`racket/path`.
Also, `path-get-extension` was intended to work on a path
that is syntactically a directory, so fix and test that.
Provide a cleaned-up set up path-extension functions. In contrast
to `path-{add,replace}-suffix` and `filename-extension`, a dot
at the beginning of a path element is not treated as an extension
separator. Also, `path-extension` returns an extension including
its separator, which is more consistent with other extension
functions.
The new `path-has-extension?` function replaces many uses of
regexp matching in the base collections.
Closes#1307
Adjusts the docs, but a future replacement (possibly with a new name
to avoid compatibility issues) should behave like the old docs,
instead.
Closes#1292
