feat: support JavaScript custom fields

[danharrin]

Description

Add JsField and the reusable HasJsRenderer concern for custom fields rendered by JavaScript modules. Filament and Livewire continue to own form state, validation, and HTTP responses. React, Vue, and Svelte are development-only dependencies for the test fixtures, not dependencies of the distributed Forms package.

Renderers load lazily by URL and receive typed props and schema utilities. The integration supports two-way state binding, PHP-driven props, deferred/live/blur/debounced updates, cleanup, and remounting. Exposed schema methods are available as named utilities in both Alpine and JavaScript fields; server-side attribute checks remain enforced.

Document application and plugin implementations, including framework examples, TypeScript, Vite/esbuild bundling, asset publication, and cross-links to the existing custom-field and plugin guides.

Visual changes

Custom JavaScript fields use the existing field wrapper, including labels, helper text, and validation messages. React, Vue, and Svelte browser fixtures render equivalent HTML and cover light and dark accessibility checks.

Liberated schemas now retain their Alpine scope in a display: contents wrapper. Custom CSS that relies on their former direct-child DOM structure may need adjustment.

Functional changes

• Code style has been fixed by running the composer cs command.
• Changes have been tested to not break existing functionality.
• Documentation is up-to-date.