message: chore(build): add exports field
  ### Related Ticket(s)
  Refs carbon-design-system/carbon-custom-elements#443.
  ### Description
  This change adds [`exports` field](https://nodejs.org/dist/latest-v14.x/docs/api/esm.html#esm_package_entry_points) in `package.json`.
  Doing so enables several CDNs that support module path mapping.
  ### Changelog
  **New**
  `exports` field in `package.json`.
diff: "main": "es/index.js",
  "module": "es/index.js",
  "typings": "es/index.d.ts",
  + "exports": {
  + "./es/": "./es/",
  + "./custom-elements.json": "./custom-elements.json",
  + "./package.json": "./package.json"
  + },
  "files": [
  "es/**/*",
  "scss/**/*",
  