[linter] enforce documenting libraries

[lishaduck]

Description

Add library-level documentation comments to all libraries.

Details

When writing libraries, also add documentation to the library directive (including creating the library directive).
This encourages people to write documentation, which show up as handy hovers for imports in IDEs.

Kind

(This template doesn't seem to around post-sdk migration, so I don't know the options.)

Bad Examples

import 'package:something/something.dart';

Good Examples

/// Do awesomeness.
library;

import 'package:something/something.dart';

Considerations

1. Gets verbose quickly.
2. Can encourage quantity over quality.