[3.x] Add optimistic prop to <Form> component

[pascalbaljet]

This PR adds an optimistic prop to the <Form> component. The callback receives both the current page props and the form data being submitted.

<template>
  <Form
    action="/todos"
    method="post"
    :optimistic="(props, data) => ({
      todos: [...props.todos, { id: Date.now(), name: data.name, done: false }],
    })"
    #default="{ processing }"
  >
    <input name="name" />
    <button type="submit" :disabled="processing">Add Todo</button>
  </Form>
</template>

The formData argument is useful because the <Form> component manages data internally from form inputs, so you don't have access to it in a closure like you would with useForm().