Add Form component ref support

[pascalbaljet]

This PR adds programmatic access to Form component methods and state via refs:

<script setup>
import { Form } from '@inertiajs/vue3'
import { ref } from 'vue'

const formRef = ref()

const submitForm = () => {
  formRef.value.submit()
}

const checkState = () => {
  console.log('Is dirty:', formRef.value.isDirty)
  console.log('Has errors:', formRef.value.hasErrors)
  console.log('Errors:', formRef.value.errors)
}
</script>

<template>
  <Form ref="formRef" action="/submit" method="post">
    <input name="email" type="email" />
    <button type="submit">Submit</button>
  </Form>
</template>