Make (re)validateOn overridable per field

Previously, `(re)validateOn` could only be set at the top of the form. However, in some cases, it'd be desirable to override this behavior (mostly because some fields already fulfill the validation from the start), so I made one. I've thought about the implementation of `setValues` but left it as-is for now. ~~If the Solid implementation gets approved, I'll also implement it for others.~~ Done!