fix CompletionFunc implementation (#2234)

The new type CompletionFunc could lead to a regression.
This commit make the new `CompletionFunc` type a type alias instead,
in case projects using Cobra have created their own similar type.

This commit also adds a test to ensure that the completion function
remains backwards-compatible.

Signed-off-by: ccoVeille <3875889+ccoVeille@users.noreply.github.com>