refactor(useForm): simplify and modernize form logic

Refactors the useForm implementation to use a Proxy-based approach for form state and methods, improving Svelte 5 runes compatibility and reducing boilerplate. Removes the InertiaFormProps interface in favor of a single InertiaFormRunes type, updates method signatures for better type safety, and streamlines state management for form data, errors, and status flags.

Author: brodienguyen