feat: add useFormsy hook as alternative to withFormsy HOC

[ThibautMarechal]

Motivation

withFormsy is a class-component HOC that predates React hooks. It works, but comes with the friction typical of HOCs:

• Prop origin opacity — readers can't tell which props come from the parent vs. formsy without reading the HOC source
• TypeScript complexity — the return type references an internal type (WrapperProps) that is not portably nameable in declaration files (see Export WrapperProps from main entry to fix TS2883 in projects with declaration emit #759)
• Wrapper hell — composing multiple HOCs creates deeply nested component trees in React DevTools
• Class component internals — withFormsy is implemented as a class component, which is an increasingly rare pattern to maintain

The modern React idiom for sharing stateful logic is a hook. Formsy already exposes FormsyContext internally and all the logic in the HOC could be expressed as a useFormsy hook.

Proposed API

import { useFormsy } from 'formsy-react';

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

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

The component is a plain function — no HOC wrapping, no prop injection, full type inference.

Design notes

• useFormsy would consume FormsyContext (already exists) to register/deregister the field, sync value, and run validations — mirroring exactly what WithFormsyWrapper does today in the class component
• name is passed directly inside the options object, not injected
• withFormsy would not be removed — it stays for backward compatibility. useFormsy is an additive, opt-in API
• The hook reuses the same validation logic already extracted in utils and validationRules

Backward compatibility

• withFormsy is unchanged
• useFormsy is a new export — no breaking change