feat: support param fn

[sigoden]

We use param fn to get default value and generate choices.

# @arg arg9=`_fn`      A positional argument with defaulf value from fn
# @arg arg10[`_fn`]    A positional argument with choices from fn
# @arg arg11![`_fn`]   A required positional argument with choices from fn

# @option    --opt12=`_fn`          A option with default value from fn
# @option    --opt13[`_fn`]         A option with choices from fn
# @option    --opt14![`_fn`]        A required option with choices from fn