[ScriptUI] add Window.findElement

[fartinmartin]

Window.findElement() is documented, but it is not present in type definitions.

This PR adds findElement(name: string): _Control | null. Although, I'm not sure if that is the preferred return type. Would we want to be explicit and list the control types?

findElement(
    name: string,
  ):
    | Button
    | Checkbox
    | DropDownList
    | EditText
    | FlashPlayer
    | Group
    | IconButton
    | Image
    | ListBox
    | Panel
    | Progressbar
    | RadioButton
    | Scrollbar
    | Slider
    | StaticText
    | TabbedPanel
    | TreeView
    | null