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/option.h b/src/option.h
index 3aaaba7..5df7f21 100644
--- a/src/option.h
+++ b/src/option.h
@@ -1114,6 +1114,10 @@
 #ifdef FEAT_TERMINAL
     , BV_TWSL
 #endif
+#ifdef FEAT_VARTABS
+    , BV_VSTS
+    , BV_VTS
+#endif
     , BV_COUNT	    /* must be the last one */
 };