Added skin types to index.d.ts

This allows for code completion of the different options of the prop in IDEs to provide greater ease-of-use than just setting the type of skin to string.

Author: taeh98

src/index.d.ts

@@ -3,7 +3,7 @@ import {StyleProp, ViewStyle} from "react-native";
 
 export interface ReactNativeInputSpinnerProps {
 	type?: string;
-	skin?: string;
+	skin?: "clean" | "modern" | "paper" | "round" | "square";
 	min?: string | number;
 	max?: string | number;
 	value?: string | number;