[nit] don't use var

* Use `const` and `let` instead of `var`. Can be added to future lint rules for this SDK and Cordova SDK

Author: nan-li

src/index.ts

@@ -183,7 +183,7 @@ export default class OneSignal {
     static promptForPushNotificationsWithUserResponse(fallbackToSettingsOrHandler?: boolean | ((response: boolean) => void), handler?: (response: boolean) => void): void {
         if (!isNativeModuleLoaded(RNOneSignal)) return;
 
-        var fallbackToSettings = false;
+        let fallbackToSettings = false;
 
         if (typeof fallbackToSettingsOrHandler === "function") {
             // Method was called like promptForPushNotificationsWithUserResponse(handler: function)