chore: update package.json exports to include TypeScript types

Author: taro-28

package.json

@@ -4,7 +4,10 @@
   "description": "Mock implementation of the Next.js Router",
   "main": "dist/index",
   "exports": {
-    ".": "./dist",
+    ".": {
+      "types": "./dist/index.d.ts",
+      "default": "./dist"
+    },
     "./*": "./dist/*",
     "./package.json": "./package.json"
   },