add: unit and integration testing

app/node_modules/leven/index.d.ts

@@ -0,0 +1,21 @@
+declare const leven: {
+	/**
+	Measure the difference between two strings.
+
+	@example
+	```
+	import leven = require('leven');
+
+	leven('cat', 'cow');
+	//=> 2
+	```
+	*/
+	(left: string, right: string): number;
+
+	// TODO: Remove this for the next major release, refactor the whole definition to:
+	// declare function leven(left: string, right: string): number;
+	// export = leven;
+	default: typeof leven;
+};
+
+export = leven;