feat: add lightweight form state helper

[landeqiming666]

Summary

Adds a lightweight render-prop form state helper with field bindings, reset and submit helpers, plus withForm().

Included

• Root package export and GitBook navigation
• Type-safe form bag and field binding interfaces
• Text and checkbox bindings, reset, submit and HOC coverage
• Storybook example and focused documentation

Validation

• yarn test
• yarn build
• git diff --check

Closes #25.

[landeqiming666]

Hi, a brief follow-up on this PR. It remains clean and mergeable. The form-state helper is kept lightweight and covered by focused behavior tests. Happy to adjust the API surface or examples if preferred.