[3.0] Fix React useForm submitting stale data after synchronous setData

[skryukov]

In the React adapter, calling setData() (or reset()) and then immediately submitting the form in the same event handler submits the pre-update data, not the value just written.

const { data, setData, post } = useForm({ code: '' })

onChange={(value) => {
  setData('code', value)
  if (value.length === 6) {
    post('/verify-otp') // submits the *previous* value ❌
  }
}}

Root cause: submit() reads form data from dataRef.current, but dataRef is only refreshed inside a post-commit useEffect. Vue and Svelte are unaffected.

See inertiajs/inertia-rails#300