Add abbreviation list mechanism

The question of abbreviations, I suggest should be handled by adopting lists of abbreviations for events and container titles. Zotero currently will automatically generate journal abbreviations from the MEDLINE list and has some hack-y methods to specify a different list. We should discuss whether a formally specified abbreviation list mechanism makes sense. I think it does.

_Originally posted by @bwiernik in https://github.com/citation-style-language/schema/pull/268#issuecomment-659661693_

I think we should absolutely add a mechanism for abbreviations. CSL 1.0.2 will improve the citation-label feature and add a classic item type. For this an abbreviation list mechanism would be useful.

[bwiernik]

The gist of what citeproc-js/pandoc-citeproc currently do:

1. Take an abbreviations file
2. Do abbreviation replacement based on that file when form="short"

The extent of support I think might be reasonable:

1. Formalize this structure:
   1. The existence structure of abbreviations files
   2. The logic for replacement based on current behavior
2. Provide an informational element in the style info indicating what abbreviations list a style uses.
   1. This wouldn’t have to be used by the citation processor, but a calling application might use it to supply different abbreviations lists, e.g., MEDLINE for medical journals but Society of Biblical Literature’s special list for that style.

[denismaier]

Ok. Then this means we formalize the current structure.

With that in mind, I’d suggest that citation-label is a problem for the calling application to solve. Either the user can enter it into the data, or the application can provide a way to fill the citation-label field for the item in the document (e.g., in Zotero, if a style calls citation-label, a text entry box could be added to the locator/suffix/prefix screen).

How do you think we should/could communicate this to calling application devs? Like: “CSL has a special variable citation-label that can be used in X and Y ways. For optimal UX please provide ways to do Z.”