Bump version to publish 3.1.0.

These changes have already rolled into the SDK, Flutter, and google3.

Author: munificent

lib/src/cli/formatter_options.dart

@@ -13,7 +13,7 @@ import 'show.dart';
 import 'summary.dart';
 
 // Note: The following line of code is modified by tool/grind.dart.
-const dartStyleVersion = '3.1.0-wip';
+const dartStyleVersion = '3.1.0';
 
 /// Global options parsed from the command line that affect how the formatter
 /// produces and uses its outputs.

pubspec.yaml

@@ -1,6 +1,6 @@
 name: dart_style
 # Note: See tool/grind.dart for how to bump the version.
-version: 3.1.0-wip
+version: 3.1.0
 description: >-
   Opinionated, automatic Dart source code formatter.
   Provides an API and a CLI tool.