reenable require_trailing_commas since trailing_commas: preserve just landed

[dkbast]

It would be great to reenable require_trailing_commas again once the next version with preserve_trailing included is released.

See: https://github.com/dart-lang/dart_style/pull/1672 https://github.com/dart-lang/dart_style/commit/71ac08514f771363b9b4b105786042d0ab91d569

[spydon]

Since this is out now it would be great to get this into the repo, the only thing that need to be added is this:

formatter:
  trailing_commas: preserve