[enhanced-parts] dart_style support

[eernstg]

[dart-github-bot]

Summary: The issue requests support for the enhanced-parts style rule in the dart_style package, which would allow users to enforce a specific style for enhanced parts in Dart code.

[lrhn]

No new formatting patterns needed.
Configurable part directives should be formatted just like conditional imports,
And imports, exports and parts should be formatted the same no matter which file they're in.