Keep useTransition backward compatibility and add startTransition (#142)

* Keep `useTransition` backward compatibility

* Add `startTransition`

Author: stephanoskomnenos

src/React.res

@@ -411,13 +411,23 @@ external displayName: component<'props> => option<string> = "displayName"
 
 // Actions
 
-type transitionFunction = unit => promise<unit>
-
+type transitionFunction = unit => unit
 type transitionStartFunction = transitionFunction => unit
 
+type transitionFunctionAsync = unit => promise<unit>
+type transitionStartFunctionAsync = transitionFunctionAsync => unit
+
+/** `startTransition` lets you render a part of the UI in the background. */
+@module("react")
+external startTransition: transitionStartFunction = "startTransition"
+@module("react")
+external startTransitionAsync: transitionStartFunctionAsync = "startTransition"
+
 /** `useTransition` is a React Hook that lets you render a part of the UI in the background. */
 @module("react")
 external useTransition: unit => (bool, transitionStartFunction) = "useTransition"
+@module("react")
+external useTransitionAsync: unit => (bool, transitionStartFunctionAsync) = "useTransition"
 
 type action<'state, 'payload> = ('state, 'payload) => promise<'state>