✨ (base): Implementa Navbar con funcionalidad base

Añade `Navbar::with_position()` y `navbar::Position` al núcleo, con las
clases de Basic para barras fijas y pegajosas, y `navbar::Item::form()`
para mostrar un formulario dentro de la barra.

Las barras sin botón de despliegue (`Simple` y `SimpleBrandLeft`) ya no
colapsan: el punto de corte no les afecta y su menú se mantiene en
línea, con salto de línea si no cabe. En Bootsier usan `navbar-expand` y
`SimpleBrandLeft` agrupa los contenidos junto a la marca.

extensions/pagetop-bootsier/tests/navbar_offcanvas.rs

@@ -0,0 +1,62 @@
+// Verifies that a `Navbar` with its content in an `Offcanvas` renders a working panel: the toggle
+// button must point to the `id` of the panel, and the panel must carry its own placement class.
+
+use pagetop::prelude::*;
+use pagetop_bootsier::theme::*;
+
+fn cx() -> Context {
+    Context::default().with_theme(&pagetop_bootsier::Bootsier)
+}
+
+fn panel(placement: bs::offcanvas::Placement) -> bs::Offcanvas {
+    bs::Offcanvas::new().with_placement(placement)
+}
+
+#[pagetop::test]
+async fn offcanvas_button_points_to_the_panel_id() {
+    let brand = || Brand::new().with_title(Lc::n("PageTop"));
+    let navbars = [
+        bs::Navbar::offcanvas(panel(bs::offcanvas::Placement::Start)),
+        bs::Navbar::offcanvas_brand_left(brand(), panel(bs::offcanvas::Placement::Start)),
+        bs::Navbar::offcanvas_brand_right(brand(), panel(bs::offcanvas::Placement::Start)),
+    ];
+
+    for mut navbar in navbars {
+        navbar = navbar.with_item(bs::navbar::Item::nav(
+            bs::Nav::new().with_item(bs::nav::Item::link(Lc::n("Home"), "/")),
+        ));
+        let html = navbar.render(&mut cx()).await.into_string();
+
+        // The panel is rendered by the navbar, so the navbar must set its `id` up.
+        let target = html
+            .split(r##"data-bs-target="#"##)
+            .nth(1)
+            .and_then(|rest| rest.split('"').next())
+            .unwrap_or_default();
+        assert!(!target.is_empty(), "the button has no target in {html}");
+        assert!(html.contains(&format!(r#"aria-controls="{target}""#)));
+        assert!(
+            html.contains(&format!(r#"id="{target}""#)),
+            "no panel with id `{target}` in {html}"
+        );
+    }
+}
+
+#[pagetop::test]
+async fn offcanvas_panel_gets_its_own_classes() {
+    for (placement, class) in [
+        (bs::offcanvas::Placement::Start, "offcanvas-start"),
+        (bs::offcanvas::Placement::End, "offcanvas-end"),
+    ] {
+        let mut navbar = bs::Navbar::offcanvas(panel(placement)).with_item(bs::navbar::Item::nav(
+            bs::Nav::new().with_item(bs::nav::Item::link(Lc::n("Home"), "/")),
+        ));
+        let html = navbar.render(&mut cx()).await.into_string();
+
+        // Without them Bootstrap would show the panel inline instead of hiding and placing it.
+        assert!(
+            html.contains(&format!(r#"class="offcanvas {class}""#)),
+            "missing `offcanvas {class}` in {html}"
+        );
+    }
+}