Add types to exports fields

Add this field to support Typescript with the new Node16 module resolution.
https://devblogs.microsoft.com/typescript/announcing-typescript-4-7/#package-json-exports-imports-and-self-referencing

Author: jigsawye

package.json

@@ -5,6 +5,7 @@
   "main": "./dist/index.js",
   "exports": {
     ".": {
+      "types": "./dist/index.d.ts",
       "require": "./dist/index.js",
       "import": "./dist/index.mjs"
     },