biblatex/biber @set entry

[flopraden]

By using biber testing data, I see @set entries

After searching, I found this:
http://ctan.mirrors.hoobly.com/macros/latex/contrib/biblatex/doc/biblatex.pdf page 113 (3.11.5.1 Static entry sets)

It could be interessting to parse it and store it.

Like cross-refs, I'm opening an issue to discuss it :)

[claell]

Draft PR #587 adds an explicit @set/entryset regression as part of a broader tested BibLaTeX structural profile: #587. The exact synthetic fixture was also accepted by Biber 2.21 data-model validation.