✨ Nice way to handle enclosings for months while writing with the default stack

Instead of `month = {1}`, I would like to see `month = jan` in my bibtex outputs. I see that `write_string` and `write_file` both have some middleware argument, but something like

```python
write_string(
    library,
    bibtex_format=bibtex_format,
    prepend_middleware=[MonthAbbreviationMiddleware(True)],
)
```

only gives

```
month = {jan}
```

(with brackets).

Any hints?