title: Remove duplicated Intellisense suggestions
body: <p>I'm writing extension that provides CompletionItems to VSCode CSS Intellisense. The issue is that some completions are already being provided to Intellisense by internal VSCode completion providers resulting in showing same complitions twice.</p>
  
  <p>What I need is to remove those duplications. How can I achieve this without messing with css language-server?</p>