Have the ability to create custom rules, closes #1162 (#2040)

* Have the ability to create custom rules, closes #1162

* Sort extensions.dart

Author: Guillergood

lib/extensions.dart

@@ -4,4 +4,7 @@ export 'src/common/utils/platform.dart';
 export 'src/common/utils/string.dart';
 export 'src/common/utils/widgets.dart';
 export 'src/document/nodes/leaf.dart';
+export 'src/rules/delete.dart';
+export 'src/rules/format.dart';
 export 'src/rules/insert.dart';
+export 'src/rules/rule.dart';