Expose yyjson_doc_get_read_size?

Hi - for reading NDJSON / JSONL files with `ReaderFlags.STOP_WHEN_DONE` - is not extremely easy as the read size that `yyjson` consumed is not available for the caller.

`yyjson_doc_get_read_size()` can do this-  Perhaps as a `.size` or `.read_bytes` on `Document` ?

Thanks