Add TypeScript definition

Author: karol-majewski

index.d.ts

@@ -0,0 +1,3 @@
+type AsyncFunction<T> = (...args: any[]) => Promise<T>;
+
+export default function greenlet<T extends AsyncFunction<U>, U>(fn: T): T;

package.json

@@ -5,6 +5,7 @@
   "source": "greenlet.js",
   "main": "dist/greenlet.js",
   "module": "dist/greenlet.m.js",
+  "types": "./index.d.ts",
   "scripts": {
     "prepare": "microbundle",
     "test": "eslint *.js && npm run -s prepare && karmatic"