[tool] Conditionalize color on stdout (#4436)

Refactors colorization to a centralized utility file, and makes it all conditional on `stdout` having ANSI escape support. This makes the output more readable on LUCI, which unlike the Cirrus log display doesn't handle ANSI.

Fixes https://github.com/flutter/flutter/issues/89392

script/tool/lib/src/update_excerpts_command.dart

@@ -9,7 +9,7 @@ import 'package:meta/meta.dart';
 import 'package:yaml/yaml.dart';
 import 'package:yaml_edit/yaml_edit.dart';
 
-import 'common/core.dart';
+import 'common/output_utils.dart';
 import 'common/package_looping_command.dart';
 import 'common/pub_utils.dart';
 import 'common/repository_package.dart';