fix: allow special chars and tab chars in options

ref: #3242

Author: lervag

autoload/vimtex/syntax/core.vim

@@ -51,6 +51,8 @@ function! vimtex#syntax#core#init_rules() abort " {{{1
   syntax cluster texClusterOpt contains=
         \texCmd,
         \texComment,
+        \texTabularChar,
+        \texSpecialChar,
         \texGroup,
         \texLength,
         \texOpt,

test/test-syntax/test-core.tex

@@ -141,4 +141,9 @@ \section*{Acknowledgement}
   %mathzone end
 }
 
+\foo[abc\\def]
+\foo[abc\%def]
+\foo[abc\\ def]
+\foo[abc\% def]
+
 \end{document}