patch 8.2.3424: a sequence of spaces is hard to see in list mode

Problem:    A sequence of spaces is hard to see in list mode.
Solution:   Add the "multispace" option to 'listchars'. (closes #8834)
diff --git a/src/structs.h b/src/structs.h
index 8a94fc1..eae6cd9 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -3376,6 +3376,7 @@
     int		tab3;
     int		trail;
     int		lead;
+    int		*multispace;
 #ifdef FEAT_CONCEAL
     int		conceal;
 #endif