Bibliography package improvements on BibTeX syntax

[Omikhleia]

Branch for some bibliography package fixes and improvements

Let's start gently and see where / how it goes.

Closes #2021 Closes #2051 Closes #2050 Closes #1860

[Omikhleia]

So this first PR is focused low-hanging fruits, regarding the support of more Bib(La)TeX syntax elements without breaking existing working behavior yet:

• Ignoring all content between entries (previous implementation mandated using % comments, but this is not correct)
• Ignoring @preamble elements (specific to TeX-based systems)
• Supporting @string elements ("abbreviations") and string concatenation with # (BibTeX compatibility)
• Supporting @xdata elements and xdata fields (Biber/BibLaTeX compatibility) for inheritance
• Supporting crossref fields with (an initial partial implementation of) the biber/biblatex data inheritance rules
• Supporting \& as legacy alternative to & for BibTeX compatibility
• Supporting ~ as convenience for non-breaking space, and \~ to get a tilde (BibTeX compatibility too)
• XML-escaping (&, <, >) in input, since we are later wrapping them in XML tags (for italics etc.) and processing the formatted output as XML

Later, I'll want to refactor some of the code and reorganize the files differently, in order to tackle properly with issues that will change the output, but this first step should be a decent base for the work to come, and it remains minimally readable (which would become harder after reshuffling some of the logic in different files).

Ready for review.

[alerque]

Just a side note on the Lua style thing.... when you see me push those kind of fixups just know there is absolutely no need to to make an effort to preserve them. If you happen to have other work in a commit or branch and my style fixups are causing rebase conflicts or anything like that just keep the work you have and blow the code formatting fixes away. They are 100% automated and next to zero work to redo at any point, so don't stress if they are causing conflicts. And don't stress about getting the formatting right while writing code either, whatever is easier to write you can write, the auto formatter can take it from any Lua code that runs.