The change provides a way to use citation keys that contain
special characters not usable with the standard citation
key syntax. Example: `@{foo_bar{x}'}` for the key `foo_bar{x}`.
Closes#6026.
The change requires adding a new parameter to the `citeKey`
parser from Text.Pandoc.Parsing [API change].
Markdown reader: recognize @{..} syntax for citatinos.
Markdown writer: use @{..} syntax for citations when needed.
Update manual with curly-brace syntax for citations.
Closes#6026.
