Support nested paths in form error types

[talelmishali]

This PR adds TypeScript support for handling nested form errors in the React and Vue packages on Form Helper (useForm)

Problem

Currently, when dealing with nested form objects, TypeScript doesn't provide proper type safety for error paths. For example:

interface CustomerForm {
    contact: {
        email: string;
    }
}

// Currently throws TS error even though Laravel returns "contact.email"
form.errors['contact.email']

Changes

@inertiajs/react

• Added FormDataKeys utility type for nested path support
• Updated error types to handle nested paths
• Modified error handling methods to support nested paths

@inertiajs/vue

• Added FormDataKeys utility type for nested path support
• Updated error types to handle nested paths
• Modified error handling methods to support nested paths

Example Usage

interface CustomerForm {
    name: string;
    contact: {
        email: string;
        phone: {
            home: string;
        }
    }
}

const form = useForm<CustomerForm>({...});

// Now TypeScript properly supports:
form.errors['contact.email']       // ✓ Valid
form.errors['contact.phone.home']  // ✓ Valid
form.errors['invalid.path']        // ✗ Type Error