fix: Forward declare the Option parseHook

[heysokam]

parseHook for Option[T] is defined at src/jsony.nim:494,
but it is missing from the forward declaration block at the top of the file.

When parseObjectInner line 389, binds parseHook for a field of type Option[T], the Option overload is not yet in scope,
overload resolution then selects parseHook*[T: object|ref object] instead, because Option is an object.
The object hook starts with eatChar(s, i, '{'), so parsing any object with an Option field can fail with:

    Expected { but got 0 instead. At offset: N

Declaring the Option hook alongside the others fixes the binding.