feat: add useFormsy hook as alternative to withFormsy HOC

[ThibautMarechal]

Closes #761

What

Adds a useFormsy<V>(options) hook that provides the same field integration as withFormsy without the HOC wrapper.

export function useFormsy<V>(options: UseFormsyOptions<V>): UseFormsyResult<V>

New exports:

• useFormsy — the hook
• UseFormsyOptions<V> — same shape as WrapperProps<V> (name, value, validations, …)
• UseFormsyResult<V> — same shape as InjectedProps<V> + value: V

Usage

import { useFormsy } from 'formsy-react';

function MyInput({ name, label }: { name: string; label: string }) {
  const { value, setValue, showError, errorMessage } = useFormsy<string>({ name });

  return (
    <div>
      <label>{label}</label>
      <input value={value ?? ''} onChange={(e) => setValue(e.target.value)} />
      {showError && <span>{String(errorMessage)}</span>}
    </div>
  );
}

With validations:

const { value, setValue, showError, errorMessage } = useFormsy<string>({
  name: 'email',
  validations: { isEmail: true },
  validationErrors: { isEmail: 'Please enter a valid email' },
  required: true,
});

How it works

useFormsy mirrors the lifecycle of the WithFormsyWrapper class component:

Class component	Hook equivalent
Constructor — seed state, setValidations	useState initial value + useEffect([], [])
componentDidMount — attachToForm	useEffect (mount)
componentWillUnmount — detachFromForm	useEffect cleanup
componentDidUpdate — sync value	useEffect([valueProp])
componentDidUpdate — sync validations	useEffect([validationsProp, requiredProp])
setState called by form context	Mutable ref + useState setter

The form context (attachToForm, validate, …) operates on a component-like ref object that forwards state reads/writes to the React state, keeping both in sync.

Backward compatibility

• withFormsy is unchanged
• useFormsy is a purely additive export
• No breaking changes

[ThibautMarechal]

This is more a proposal. If you agree that it's a good idea, I'll be happy to work on it in order to add tests and share more common code between the hoc & hook :)