Apply directives_ordering to part

[FMorschel]

Describe the issue
There is the directives_ordering. It does not apply to part, only import and export.

To Reproduce

• Create a simple project.
• Ensure directives_ordering is being used:

linter:
  rules:
    directives_ordering: true

• Create 2 new files a.dart and b.dart inside lib.
• Make them part of main.dart:

part 'b.dart';
part 'a.dart';

Expected behavior
To trigger directives_ordering lint.

Additional context
If you disagree with adding this rule to directives_ordering, then maybe we could create a new linter to that.

[bwilkerson]

It does seem reasonable to me that the lint should check part directives.