Attach library doc comments to library directives DRT-W1084

Attach library doc comments (with ///) to library directives, rather than leaving them dangling near the top of a library.

BAD:

/// This is a great library.
import 'package:math';

/// This is a great library.

class C {}

GOOD:

/// This is a great library.
library;

import 'package:math';

class C {}

NOTE: An unnamed library, like library; above, is only supported in Dart 2.19 and later. Code which might run in earlier versions of Dart will need to provide a name in the library directive.