Autofocus input

[kevin-on]

Implemented custom AutoFocusPlugin.

Problem in Lexical's AutoFocusPlugin

if (rootElement !== null && (activeElement === null || !rootElement.contains(activeElement))) {
  // Note: preventScroll won't work in Webkit.
  rootElement.focus({
    preventScroll: true
  });
}

• In our use case, activeElement (MarkdownView) doesn't contain rootElement (ChatUserInput)