Update ArgLenAt Dash to search for first option with single dash instead of only double dash

[mpuckett159]

ArgLentAtDash currently only looks for the first instance of -- for an option (ie. only for "long" options) and I believe that this functionality should work for - as well, since the intention is to find the first instance of an option passed by the CLI, not just the first "long" option. For compatibility this should probably be a new function since it would have a lot of downstream implications probably. I'm happy to make the change but I wanted to see if this is something that would be accepted before writing the code.

Originally I wanted to make this change in the pflag repo but it's been 3.5 years since the last update on main so I figured I'd ask here since it seems more active.