[proposal] Enable `default` outside switch case

```dart
class PermissionStatus {
static final granted = PermissionStatus._('granted');
static final denied = PermissionStatus._('denied');
static final default = PermissionStatus._('default');

PermissionStatus._(this.value);

final String value;
}
```

'default' can't be used as an identifier because it's a keyword.
Try renaming this to be an identifier that isn't a keyword.dart(expected_identifier_but_got_keyword)

Start by reviewing Dart's keyword and identifier rules, focusing on why `default` is restricted to switch cases. Define the allowed contexts and compatibility implications for using it as an identifier, then document the language change and its expected behavior.

Written by the indexing model from the issue text.