Add prefix and suffix to Citation (group)

[jgm]

Currently citationItem has citationItemPrefix/Suffix, but there is no way to provide a prefix or suffix for the whole Citation (group). For motivation see https://github.com/jgm/pandoc/issues/10894.

Proposal:

• Add citationPrefix :: Maybe a, citationSuffix :: Maybe a to Citation a.
• Modify Eval so that these are added to the rendered citationItems.
• Add a test where citations are sorted, changing their order, to make sure that the citationPrefix and citationSuffix remain at the beginning/end of the rendered Citation.

[jgm]

Working version in branch issue156