Fixed missing braces around passed on key values

[muzimuzhi]

The received values of some keys or options may contain unescaped comma (,) and/or equal (=) characters which may break key-value processing, for example breakbefore, breakafter, and break...symbol(pre|post) options.

So in user input, such values need to be enclosed in a pair of braces, e.g., breakafter={,]}. Those braces are then removed in the extracted values (,]). Then to pass on them to other keys/options, the values need to be braced again.

This PR adds the required braces around forwarded key values, and updates breakbefore and breakafter docs to sync with gpoore/fvextra@16de52d (breakbefore and breakafter now support the escaped comma , (closes #15), 2022-11-10).

Example

\documentclass{article}
\usepackage{minted}
\setminted{breakafter={,]}} % ! Package keyval Error: ] undefined.
\setminted{breakafter={,\]}} % ! Missing \endcsname inserted. <to be read again> \protect
\begin{document}
\end{document}

• Before: error(s) raised.
• After: no errors.

[muzimuzhi]

To catch all similar cases, I've manually reviewed all uses of = in minted.dtx.

Force pushing rebased commits.