Escaping and brace nesting in bibtex field values

[Omikhleia]

Extracted from review comment https://github.com/sile-typesetter/sile/pull/2048#discussion_r1642004205

BibTeX allows two types syntax for (leaf-)string values in field:

• Braced {My Awesome Book}
• Quoted "My Awesome Book"

Out bibtex implementation currently supports both on the surface, but things get much trickier inside these strings:

• There can be nested braced content with a specific/mixed meaning (making TeX commands "robust", enforcing by-passing casing rules, or even used as a separator... Ex:
  • {{\relax Ph}ilip Doe}, {{\LaTeX}} etc. = some TeX-based mess
  • {Doe, John and {National Aeronautics and Space Administration}} = avoid the organization being seen and parsed as a name (split at and)
  • (In addition to name fields, in BibLaTeX) special handing of multiple institution, publisher, location fields, ex. {Johnson {and} Smith} vs London and Paris
  • {My {A}we{S}ome Book} when lower-cased, title-cased, or whatever, would keep A and S capitals...
• In the quoted variant, we currently make the assumption that backslash-escaping quotes should work, e.g. "My \"Awesome\" Book", but the official way documented in "Tame the BeaST. The B to X of BibTEX,* version 1.4 (2009) seems to be "My {"}Awesome{"} Book"

--> For full correct support, we may need to revise our grammar at some point (and or consolidate some entries differently).