feat: add new BackgroundHandler type (#364)

Fixes netlify/docs#2328

`Promise<void>` is a valid return type for a function.

example:
```
const handler: Handler = async () => {
  return;
}
```

I also added `tsd` to test for the types.

.eslintrc.js

@@ -12,6 +12,7 @@ module.exports = {
       rules: {
         'no-magic-numbers': 'off',
         'promise/prefer-await-to-callbacks': 'off',
+        'unicorn/filename-case': 'off',
       },
     },
   ],