Use position: relative for .line-number elements

...because packages like `.git-diff` are relying on this behavior to
position their decorations. This didn't seem to degrade layout times, so
it makes sense to just add it to keep package breakage to a minimum.

static/text-editor.less

@@ -18,6 +18,7 @@ atom-text-editor {
     padding-left: .5em;
     white-space: nowrap;
     opacity: 0.6;
+    position: relative;
 
     .icon-right {
       .octicon(chevron-down, 0.8em);