Clic: allow param and stop at same level

Context

The Clic library forbids to define commands with the same prefix where one is a strict prefix of the other, unless the first one does not have any options. This is to prevent conflicts between an option and a parameter, but this restriction is not necessary as options and parameters can be differentiated.

This MR removes this restriction.

Prerequisite for !8547 (merged).

Manually testing the MR

dune exec src/lib_clic/test/main.exe