Lint to split comments when > line length

[pq]

Migrated from dartbug.com/22003:

I ran dartformat like this:

dartformat -l 58 -w *.dart

The input file included these two lines:

  // _cache is library-private, thanks to the _ in front of its name.
  static final Map<String, Logger> _cache = <String, Logger>{};

The output for those lines was this:

  // _cache is library-private, thanks to the _ in front of its name.
  static final Map<String, Logger> _cache =
      <String, Logger>{};

I expected something like this:

  // _cache is library-private, thanks to the _ in front
  // of its name.
  static final Map<String, Logger> _cache =
      <String, Logger>{};

[pq]

Closing in favor of: #57705.