✨ (pagetop): Añade with_expand() a Navbar

Navbar guarda el punto de corte a partir del cual deja de colapsar
(`Breakpoint::Md` por defecto) y añade la clase `navbar-expand-{nombre}`
o `navbar-expand` si no tiene ancho mínimo. El nombre lo resuelve el
tema activo con `Theme::breakpoint_entry()`.

tests/component_navbar.rs

@@ -26,3 +26,45 @@ async fn push_end_adds_an_automatic_start_margin() {
     assert!(html.contains("_flex-item-offset_auto_"));
     assert!(assets.contains("_flex-item-offset_auto_{margin-inline-start:auto}"));
 }
+
+// **< Navbar::with_expand >************************************************************************
+
+#[pagetop::test]
+async fn expand_is_md_by_default_and_can_be_set() {
+    assert_eq!(Navbar::simple().expand(), Breakpoint::Md);
+    assert_eq!(
+        Navbar::simple().with_expand(Breakpoint::Lg).expand(),
+        Breakpoint::Lg
+    );
+}
+
+#[pagetop::test]
+async fn expand_class_is_md_by_default() {
+    let mut navbar = Navbar::simple_toggle().with_item(navbar::Item::nav(one_link_nav()));
+    let html = navbar.render(&mut Context::default()).await.into_string();
+
+    assert!(html.contains("navbar-expand-md"));
+}
+
+#[pagetop::test]
+async fn expand_class_uses_the_breakpoint_name() {
+    let mut navbar = Navbar::simple_toggle()
+        .with_expand(Breakpoint::Xl)
+        .with_item(navbar::Item::nav(one_link_nav()));
+    let html = navbar.render(&mut Context::default()).await.into_string();
+
+    assert!(html.contains("navbar-expand-xl"));
+    assert!(!html.contains("navbar-expand-md"));
+}
+
+#[pagetop::test]
+async fn expand_class_has_no_name_without_a_breakpoint_width() {
+    // `Xs` has no minimum width, so the class carries no breakpoint name.
+    let mut navbar = Navbar::simple_toggle()
+        .with_expand(Breakpoint::Xs)
+        .with_item(navbar::Item::nav(one_link_nav()));
+    let html = navbar.render(&mut Context::default()).await.into_string();
+
+    assert!(html.contains("navbar-expand"));
+    assert!(!html.contains("navbar-expand-"));
+}