provide missing LaTeX macro, e.g. for AFP/PAC_Checker;

--- a/lib/texinputs/isabellesym.sty	Mon Sep 26 11:30:44 2022 +0200
+++ b/lib/texinputs/isabellesym.sty	Mon Sep 26 20:40:19 2022 +0200
@@ -480,6 +480,7 @@
 \newcommand{\isactrlundefined}{\isakeywordcontrol{undefined}}
 \newcommand{\isactrltvar}{\isakeywordcontrol{tvar}}
 \newcommand{\isactrlvar}{\isakeywordcontrol{var}}
+\newcommand{\isactrlverbatim}{\isakeywordcontrol{verbatim}}
 \newcommand{\isactrlConst}{\isakeywordcontrol{Const}}
 \newcommand{\isactrlConstUNDERSCORE}{\isakeywordcontrol{Const{\isacharunderscore}}}
 \newcommand{\isactrlConstUNDERSCOREfn}{\isakeywordcontrol{Const{\isacharunderscore}fn}}