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/runtime/optwin.vim b/runtime/optwin.vim
index cd1bea0..5ab5792 100644
--- a/runtime/optwin.vim
+++ b/runtime/optwin.vim
@@ -856,6 +856,14 @@
 call append("$", "shiftwidth\tnumber of spaces used for each step of (auto)indent")
 call append("$", "\t(local to buffer)")
 call <SID>OptionL("sw")
+if has("vartabs")
+  call append("$", "vartabstop\tlist of number of spaces a tab counts for")
+  call append("$", "\t(local to buffer)")
+  call <SID>OptionL("vts")
+  call append("$", "varsofttabstop\tlist of number of spaces a soft tabsstop counts for")
+  call append("$", "\t(local to buffer)")
+  call <SID>OptionL("vsts")
+endif
 call append("$", "smarttab\ta <Tab> in an indent inserts 'shiftwidth' spaces")
 call <SID>BinOptionG("sta", &sta)
 call append("$", "softtabstop\tif non-zero, number of spaces to insert for a <Tab>")