Add types in package exports (#10)

Now typescript supports special "types" type in package.json#exports
https://devblogs.microsoft.com/typescript/announcing-typescript-4-5-beta/#packagejson-exports-imports-and-self-referencing

package.json

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