Character strings that are only digits are not getting wrapped in single quotes if they end in 8 or 9

This has downsteam impacts on other packages, noteably quarto in my case.  If the single quotes are missing, the string ends up getting converted to a numeric.

```
> yaml::as.yaml("027")
[1] "'027'\n"
> yaml::as.yaml("028")
[1] "028\n"
```