Update package.json: add types to exports too (#46)

Based on https://stackoverflow.com/questions/76211877/the-xxxx-library-may-need-to-update-its-package-json-or-typings-ts

package.json

@@ -30,6 +30,7 @@
   "main": "public",
   "exports": {
     ".": {
+      "types": "./public/index.d.ts",
       "import": "./public/index.mjs",
       "require": "./public/index.js"
     },