Make fish_opt option optional

[oyarsa]

#7585 seems to have fixed only the string spec for argparse, but fish_opt still requires --short to be provided, even when we have --long-only:

String spec version:

# script.fish
argparse 'opt=' -- $argv
echo $_flag_opt

Output:

❯ fish script.fish --opt hi
hi

fish_opt version:

# script2.fish
set option (fish_opt --long=opt --long-only --required)
argparse $option -- $argv
echo $_flag_opt

Output:

❯ fish script2.fish --opt hi
fish_opt: The --short flag is required and must be a single character
argparse: --opt: unknown option

I expected fish_opt to have same capabilities as the string spec.

Fish version: 3.6.1

[faho]

To be honest: Use argparse directly.

I don't think fish_opt is really all that helpful. Personally, I consider it a mistake and would prefer to remove it.