chore: add types to exports field to be compatible with nodenext module resolution (#1522)

See [1] for detail, in `nodenext` module resolution it requires a
`types` field in `exports` with full filename including extension.

[1]: microsoft/TypeScript#46770 (comment)

Reference: https://www.typescriptlang.org/tsconfig/#module

Author: otakustay

package.json

@@ -26,7 +26,8 @@
         "node": "./build/esm-debug/index.js",
         "default": "./build/esm/index.js"
       },
-      "require": "./build/cjs/index.js"
+      "require": "./build/cjs/index.js",
+      "types": "./build/esm/index.d.ts"
     }
   },
   "types": "./build/esm/index.d.ts",