feat(ui): add 'winbar'

Adds support for a bar at the top of each window, enabled through the
`'winbar'` option.

Co-authored-by: Björn Linse <bjorn.linse@gmail.com>

src/nvim/api/keysets.lua

@@ -80,6 +80,7 @@ return {
     "maxwidth";
     "fillchar";
     "highlights";
+    "use_winbar";
     "use_tabline";
   };
   option = {