Bibtex reader chokes at @preamble and @string

In (legacy) bibtex files, `@preamble` and `@string` are special entry types

- `@preamble` can contain (La)TeX instructions...
- `@string` allows defining abbreviations for use in other entries

Our reader errors if one or the other is present.
We should at best:
- Skip `@preamble`
- Manage `@string` or gracefully degrade if we don't want to support it.