Add types manifest field (#391)

This field is recommended by the TypeScript documentation about
publishing [1]. It helps consumers find the type declarations.

[1]: https://www.typescriptlang.org/docs/handbook/declaration-files/publishing.html

package.json

@@ -7,6 +7,7 @@
     "dist/"
   ],
   "main": "./dist/index.js",
+  "types": "./dist/index.d.ts",
   "publishConfig": {
     "registry": "https://registry.npmjs.org/",
     "access": "public"