title: Typescript: transform required properties to optional
body: <p>How can non-optional properties be transformed to optional ones?
  Here's the code:</p>
  
  <pre><code>interface Foo {
    bar: any // no '?', hence this prop is required
  }
  
  type KeysOfFoo = {
    [K in keyof Foo]: any
  }
  
  const keysOfFoo: KeysOfFoo = {} // No tsc error wanted here, got: "type '{}' is not assignable to type KeysOfFoo"
  </code></pre>