title: ESLint rule to allow only either default or named exports per file
body: <p>I wish to have a following ESLint rule:</p>
  
  <p>A js file can have either one default export, or as many named exports as possible. But not both. So having one default export + named exports should result in an eslint warning. Is this possible with eslint currently? If not, would it be easy to make a rule like that?</p>