Generalize the protection of brackets in citation arguments

Generalize the protection of brackets in citation arguments

Whenever an argument delimiter is used inside the argument, the argument
needs to be grouped, that is

\cites({text (text) text})

or

\cite[{text [text] text}]

This fixes the original case reported in #2751 which is independent
from the general issue that the pre- and postnote field take literal
code.