Function NavItem 

fn NavItem(props: NavItemProps) -> impl IntoView

单个桌面导航项：当前项内含 .nav-indicator 胶囊（转场时只有胶囊在栏目间滑动，文字留在原位）。

Required Props

• route: Route
• label: [&'static str]
• is_active: impl Into<Signal<bool>>