Fix types field to be compatible with nodenext module resolution

See microsoft/TypeScript#46770 (comment) for detail, in `nodenext` module resolution it requires a full filename including extension

package.json

@@ -10,6 +10,7 @@
 		"email": "[email protected]",
 		"url": "https://sindresorhus.com"
 	},
+	"types": "index.d.ts",
 	"engines": {
 		"node": ">=6"
 	},