support NodeJS ESM Module only project

Currently, ESM Module only project doesn't work with size-limit, both esbuild and webpack plugin fails to recognize the project:

// package.json
{
  "type": "module",
  "exports": {
    ".": {
      "import": "./lib/index.js"
    }
  }
}

They both try to look for <proj>/index.js because main was not specified.