home: vim: completion: activate 'ghost_text'

I'm still thinking whether I would want to use one of those signature
helper windows to go with this/replace this feature.

Could be especially useful in C++.

home/vim/plugin/settings/completion.vim

@@ -57,5 +57,8 @@ cmp.setup({
             cmp.config.compare.order,
         },
     },
+    experimental = {
+        ghost_text = true,
+    },
 })
 EOF