Rename package/repo to formatter

[seaneagan]

This package/repo essentially provides a "formatter for dart". dart_style doesn't really convey that, and formatter would be more consistent with the linter package.

[munificent]

I think the package name should have "dart" in it, since it's specific to styling Dart code. (This is different from the guideline to avoid putting "dart" in a package name because the package is implemented in Dart. In this case, "dart" means "this package is for working with Dart code".)

I could change the name to dart_formatter or dartfmt, but I'm not sure if it's worth the effort. The former is more precise, but it's still different from the command name so doesn't really clarify much. The latter matches the command name, but is now an ugly name.

I still wouldn't be surprised if we ended up with more functionality than just the formatter in here, so I don't know if it adds value to tighten the name today.

[seaneagan]

Came up here:

google/grinder.dart#168 (comment)