feat(segment): border to top instead of bottom for vertical variant

This makes it easier to remove the border if needed, because you're able to target the second segment with + in CSS.

For example, in case you have 2 adjacent vertical segments with a custom class of 'white', you want to remove the border and top padding to make them appear as 1. This currently requires a few overrides, but with a top border this would become:

.ui.vertical.segment.white + .white {
    border-top: none;
    padding-top: 0;
}

Author: hugopeek

src/definitions/elements/segment.less

@@ -49,10 +49,10 @@
     border-radius: 0;
     box-shadow: none;
     border: none;
-    border-bottom: @borderWidth solid @borderColor;
+    border-top: @borderWidth solid @borderColor;
   }
-  .ui.vertical.segment:last-child {
-    border-bottom: none;
+  .ui.vertical.segment:first-child {
+    border-top: none;
   }
 }