Emit the user-defined value terminator into the zsh completion pattern
to avoid doubled rest-arguments definitions:
$ my-app <TAB>
_arguments:comparguments:325: doubled rest argument definition:
*::second -- second set of of multi-length arguments:
https://github.com/clap-rs/clap/issues/3266#issuecomment-1007901407
noted that including the value terminator is one step towards a
robust solution for handling multiple multi-valued arguments.
This change does not yet handle automatically detecting when a value
terminator is needed, but it does add tests to ensure that
user-specified value terminators are used on zsh.
Related-to: #3022
Signed-off-by: David Aguilar <davvid@gmail.com>