Changes
### Added
- Support the `readonly` modifier on class and trait properties: `readonly` is a property modifier keyword that appears in the property `visibility` list (e.g. `public readonly int x`), independent of position relative to `public`/`protected`/`private` and combining with every property type prefix (builtin, `?type`, `` cast and union). The parser only records the modifier; the typed / no-default / no-static rules are enforced by the compiler ([zephir-lang/zephir#2614](https://github.com/zephir-lang/zephir/issues/2614)).
### Changed
- `readonly` is now a reserved keyword; identifiers named `readonly` are a syntax error.