Fix incompatible ref typing with ReactElement (& popular react libraries) (#2099)

* Update index.d.ts

* Update index.d.ts

Co-authored-by: Jovi De Croock <[email protected]>

Author: xiel

src/index.d.ts

@@ -14,7 +14,11 @@ declare namespace preact {
 		type: ComponentType<P> | string;
 		props: P & { children: ComponentChildren };
 		key: Key;
-		ref: Ref<any> | null;
+		/**
+		 * ref is not guaranteed by React.ReactElement, for compatiblity reasons
+		 * with popular react libs we define it as optional too
+		*/
+		ref?: Ref<any> | null;
 		/**
 		 * The time this `vnode` started rendering. Will only be set when
 		 * the devtools are attached.