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)

src/option.h

@@ -1113,6 +1113,10 @@ enum
     , BV_WM
 #ifdef FEAT_TERMINAL
     , BV_TWSL
+#endif
+#ifdef FEAT_VARTABS
+    , BV_VSTS
+    , BV_VTS
 #endif
     , BV_COUNT	    /* must be the last one */
 };