patch 8.1.0105: all tab stops are the same

Problem:    All tab stops are the same.
Solution:   Add the variable tabstop feature. (Christian Brabandt,
            closes #2711)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 7325f5d..0cd7527 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -6436,6 +6436,9 @@
 	"user-commands",    /* was accidentally included in 5.4 */
 	"user_commands",
 #endif
+#ifdef FEAT_VARTABS
+	"vartabs",
+#endif
 #ifdef FEAT_VIMINFO
 	"viminfo",
 #endif