add missing types attribute

Author: heidkaemper

package.json

@@ -20,6 +20,7 @@
     "main": "dist/index.cjs.js",
     "module": "dist/index.esm.js",
     "unpkg": "dist/observer.min.js",
+    "types": "dist/index.d.ts",
     "files": [
         "/dist"
     ],