fix: include types in export

When a package is being loaded via native ESM with TypeScript in
`node16` mode, it looks up the types from the `export` property in
package.json
Defining a `types` property in `exports` allows for lookup of the
typings

see: https://www.typescriptlang.org/docs/handbook/esm-node.html#packagejson-exports-imports-and-self-referencing

package.json

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