title: Change title of untitled tab in Visual Studio Code
body: <p>I'm building a VS Code extension which includes changing the name/title of <code>untitled-1</code> tab (unsaved file).<br>
  I tried running the below code in debugger console of extension but it didn't reflect in the editor:</p>
  
  <pre class="lang-ts prettyprint-override"><code>vscode.workspace.textDocuments[0].fileName="myFile"
  </code></pre>
  
  <p>Is it not possible or am I missing something?</p>