Toolbox => Add isString function

helpers/Toolbox.ts

@@ -51,6 +51,10 @@ class Toolbox {
     public getKeysWithPrefix(obj: object, prefix: string): Array<string> {
         return Object.keys(obj).filter(key => key.startsWith(prefix));
     }
+
+    public isString(value: unknown): value is string {
+        return typeof value === 'string' || value instanceof String;
+    }
 }