feat: support skipping choice fn validation

[sigoden]

Prepend ? to choices_fn to skip validate choice value

# @option --foo1[`_fn_foo`]
# @option --foo2[?`_fn_foo`]

_fn_foo() {
    echo a1
    echo a2
}

Run test.sh --foo1 aa

error: invalid value 'aa' for '<FOO1>'
  [possible values: a1, a2]

For more information, try '--help'.

Run test.sh --foo2 aa

argc_foo2=aa