feat: strict types for form events

[ivan-demchenko]

This PR makes use of Js.Json.t instead of object type for form events. It addresses this issue: https://github.com/rescript-lang/rescript-react/issues/105

Context:

let targetValueToInt = (target: Js.Json.t): int => {
 // Js.Json.decodeObject(target)) 
 // and so on
}
// ...
<input
  onChange={
    evt => {
      setIntValue(_prev => evt->ReactEvent.Form.target->targetValueToInt);
    }
  }
/>

[mununki]

Thank you for PR! The big difference between object and JSON is that undefined doesn't exist in JSON's specification. I'm not sure why we need to type-define form event as JSON format instead of object, can you explain further?