Feature request: new render prop `handleSubmitSync`

### Are you submitting a **bug report** or a **feature request**?

Feature request

### What is the current behavior?

This code

```

{props => (

... fields go here...

Submit

)}

```

conflicts with `eslint` rule `@typescript-eslint/no-misused-promises`

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-misused-promises.md

### What is the expected behavior?

I suggest to add new render prop `handleSubmitSync` that wraps `handleSubmit` and always returns `undefined`.

### What's your environment?

```
react@18.2.0
final-form@4.20.7
react-final-form@6.5.9
eslint@8.21.0
@typescript-eslint/eslint-plugin@5.32.0
```

Search the render-prop API entry point for the existing handleSubmit implementation and inspect how its return value is handled. Add the requested handleSubmitSync behavior so it always returns undefined while preserving submission, then verify the ESLint no-misused-promises use case and existing submit behavior.

Written by the indexing model from the issue text.