Add support for dangling comma

I’d like to request support for dangling commas in `.arb` files. While I understand that ARB is based on JSON, many modern JSON parsers and linters allow for trailing commas (like [json5](https://github.com/json5/json5)), and they bring several practical benefits:

- Cleaner diffs in version control
- Avoiding unnecessary merge conflicts
- Easier editing and reordering of entries

Would it be possible to support dangling commas in the ARB spec and in the tools that consume ARB files?

Let me know if this is something you’d consider or if there’s a reason it’s intentionally disallowed.

Thanks again!