Add exports field for node

While this library targets the browser, it is sometimes helpful to use
it in need for experiments.
For example: #588

package.json

@@ -5,6 +5,7 @@
   "main": "build/main/index.js",
   "typings": "build/main/index.d.ts",
   "module": "build/esm/index.js",
+  "exports": "./",
   "repository": "https://github.com/status-im/js-waku",
   "license": "MIT OR Apache-2.0",
   "keywords": [