Add Form Context Support

[laserhybiz]

This PR adds the ability to inject form state and methods into descendant components using Vue's provide/inject API.

This is useful when using wrapper components which encapsulates form functionalities.

Before:
Form state is only accessible via slot props or template refs and has to be passed down to children

<!-- Parent.vue -->
<Form
  action="/users"
  method="post"
  #default="{
    errors,
  }"
>
  <Input name="name" :error="errors.name" />
</Form>

<!-- Input.vue -->
<script setup>
defineProps({
  name: {
    type: String,
    required: true,
  },
  error: {
    type: String,
    default: null,
  },
})
</script>

<template>
  <input type="text" :name="name" />

  <div v-if="error">
    {{ error }}
  </div>
</template>

After:
All form state is accessible in any descendant component

<!-- Parent.vue -->
<Form
  action="/users"
  method="post"
>
  <Input name="name" />
</Form>

<!-- Input.vue -->
<script setup>
import { useFormContext } from '@inertiajs/vue3'

defineProps({
  name: {
    type: String,
    required: true,
  },
})

const form = useFormContext()
</script>

<template>
  <input type="text" :name="name" />

  <div v-if="form.errors[name]">
    {{ form.errors[name] }}
  </div>
</template>

The useFormContext hook returns the same interface as the Form's slot props and template ref, including all state (errors, isDirty, processing, etc.) and methods (submit, reset, clearErrors, etc.).

I'll add the tests if this is approved.

[laserhybiz]

I guess I will also have to add this to the other adapters if it's approved.