Preserve trailing commas

[spydon]

After the new formatter was introduced it started removing trailing commas. Trailing commas is a feature which most of the Flutter & Dart community like since it makes the git diffs less noisy and the code easier to read.

My suggestion in that we add trailing_commas: preserve to the set of lint rules, so that the people depending on this package can continue to enjoy their trailing commas.

This is what would need to be added:

formatter:
  trailing_commas: preserve

[github-actions[bot]]

Link to Pyrus task