[feat] add api to get focus window like electron BrowserWindow.getFocusedWindow

[suxiaoshao]

Describe the problem

As a Tauri developer, I would like to have a way to retrieve the currently focused window, so that I can perform operations on that window, such as let it as parent_window create child window.

Describe the solution you'd like

javascript

const focusedWindow = WebviewWindow.getFocusWindow()

rust

let id=Window::get_focus_window();
let window = WindowBuilder::new(app, "main", tauri::WindowUrl::App("index.html".into()))
        .parent_window(id)
        .build()?;

Alternatives considered

No response

Additional context

No response