Add @formAssets directive

[aerni]

Using the assets field in a wizard form lead to some unexpected behavior, as noted in #86. This PR combats that issue by bundling the Filepond styles and providing a new @formAssets blade directive to simply include all scripts and styles in one go. Make sure to update your views to benefit from this.

In your form view, e.g. default.blade.php:

- @assets
-     <script type="module" src="/vendor/livewire-forms/js/livewire-forms.js"></script>
- @endassets

+ @formAssets

In your assets field view, e.g. /default/fields/assets.blade.php:

- @assets
-     <link href="https://unpkg.com/filepond@^4/dist/filepond.css" rel="stylesheet" />
-     <link href="https://unpkg.com/filepond-plugin-image-preview/dist/filepond-plugin-image-preview.css" rel="stylesheet" />
- @endassets