keep useTransition for sync and add an useTransitionAsync

this allows to remove a breaking change from v0.14.0

Author: tsnobip

src/React.res

@@ -411,14 +411,22 @@ external displayName: component<'props> => option<string> = "displayName"
 
 // Actions
 
-type transitionFunction = unit => promise<unit>
+type transitionFunction = unit => unit
 
 type transitionStartFunction = transitionFunction => unit
 
 /** `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"
 
+type transitionAsyncFunction = unit => promise<unit>
+
+type transitionAsyncStartFunction = transitionAsyncFunction => unit
+
+/** `useTransitionAsync` is a React Hook that lets you render a part of the UI in the background. */
+@module("react")
+external useTransitionAsync: unit => (bool, transitionAsyncStartFunction) = "useTransition"
+
 type action<'state, 'payload> = ('state, 'payload) => promise<'state>
 
 type formAction<'formData> = 'formData => promise<unit>