Fix #1907. Scan \DeclarePairedDelimiter for preview
Fix #1907. Scan \DeclarePairedDelimiter for preview

As DeclarePairedDelimiter is not understood by mathjax, we refactor the
declaration into a new command. Note that we only consider

\DeclarePairedDelimiter{\name}{...}(...}

The braces are mandatory around the command name.

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