fix(typescript): Moved typescript definition at the root of the project

https://www.typescriptlang.org/docs/handbook/declaration-files/publishing.html#including-declarations-in-your-npm-package

Author: Dimitri Kopriwa

src/index.d.ts renamed to index.d.ts

@@ -1,4 +1,4 @@
-const exported = require('../index');
+const exported = require('../..');
 
 describe('text exports', () => {
   Object.keys(exported).forEach((e) => {