[REQ] Allow additional info to be set for literature referencing

[mhatzl]

Describe the solution you'd like

Citations using CSL allow to optionally specify page ranges, chapters, ... for a citation.
See Cite-Items.

It should be possible to add this info after an ID is set, because this information is specific to an ID.

Possible syntax:

[&&my-id{ locator: "123"; label: "page"}, other-id]

The info is set using the same syntax as with attributes.

Describe alternatives you've considered

Alternative syntax using () instead of {}, or JSON instead of CSS-like...
Would be less consistent with other elements.

Additional context

position and near-note properties should be set automatically by Unimarkup.
For this, the citation IDs must probably be stored during rendering to validate if a citation has already been cited.