Out of bounds array access

[kalkwarf]

After installing this extension, attempting to use gh copilot suggest via the ghcs alias (for interactive mode) reports an error:

~/.local/share/gh/extensions/gh-fish/gh-copilot-alias.fish (line 37): array indices start at 1, not 0.
        switch $argv_copy[$i]
                           ^

in function 'ghcs'

I'd guess we need to preflight the argument count before trying to copy them out, but I'm not confident I understand the script well enough to make the change.