title: TypeScript pick call signature from function interface
body: <p>There is a function interface with statics:</p>
  
  <pre><code>interface MyFunction {
    (value: string): string;
    a: string;
    b: string;
  }
  </code></pre>
  
  <p>How do I <code>Pick</code> the call signature only (ignore <code>a</code> and <code>b</code>)?</p>