✨ (pagetop): Añade Flex/FlexItem para usar Flexbox

Container y Navbar lo adoptan vía `with_flex()`/`PropsOp::flex_item()`.
Y se elimina `ButtonSet` porque su funcionalidad queda cubierta por este
mecanismo más general. Incluye el ejemplo `examples/intro-flex.rs` con
los patrones de uso.

tests/component_navbar.rs

@@ -0,0 +1,68 @@
+use pagetop::prelude::*;
+
+fn one_link_nav() -> Nav {
+    Nav::new().with_item(nav::Item::link(Lc::n("Home"), "/"))
+}
+
+// **< Navbar >*************************************************************************************
+
+#[pagetop::test]
+async fn is_not_rendered_when_empty() {
+    let mut navbar = Navbar::simple();
+    let html = navbar.render(&mut Context::default()).await;
+
+    assert!(html.is_empty());
+}
+
+#[pagetop::test]
+async fn nav_root_class_is_unaffected_by_content_flex() {
+    let mut navbar = Navbar::simple()
+        .with_flex(Flex::row().with_justify(flex::ContentJustify::End))
+        .with_item(navbar::Item::nav(one_link_nav()));
+    let html = navbar.render(&mut Context::default()).await.into_string();
+
+    assert!(html.contains(r#"class="navbar""#));
+}
+
+// **< Navbar + Flex (content area) >***************************************************************
+
+#[pagetop::test]
+async fn without_flex_content_area_has_no_style_attribute() {
+    let mut navbar = Navbar::simple().with_item(navbar::Item::nav(one_link_nav()));
+    let html = navbar.render(&mut Context::default()).await.into_string();
+
+    assert!(html.contains(r#"class="navbar-content""#));
+    assert!(!html.contains("style="));
+}
+
+#[pagetop::test]
+async fn flex_adds_its_styles_to_the_content_area() {
+    let mut navbar = Navbar::simple()
+        .with_flex(Flex::row().with_justify(flex::ContentJustify::End))
+        .with_item(navbar::Item::nav(one_link_nav()));
+    let html = navbar.render(&mut Context::default()).await.into_string();
+
+    assert!(html.contains(r#"class="navbar-content""#));
+    assert!(html.contains("display: flex"));
+    assert!(html.contains("justify-content: flex-end"));
+}
+
+#[pagetop::test]
+async fn flex_gap_adds_a_style_to_the_content_area() {
+    let mut navbar = Navbar::simple()
+        .with_flex(Flex::row().with_gap(flex::Gap::Both(UnitValue::RelRem(0.5))))
+        .with_item(navbar::Item::nav(one_link_nav()));
+    let html = navbar.render(&mut Context::default()).await.into_string();
+
+    assert!(html.contains("gap: 0.5rem"));
+}
+
+// **< Navbar + FlexItem::push_end >****************************************************************
+
+#[pagetop::test]
+async fn push_end_adds_an_automatic_start_margin() {
+    let mut nav = one_link_nav().with_prop(FlexItem::push_end());
+    let html = nav.render(&mut Context::default()).await.into_string();
+
+    assert!(html.contains(r#"style="margin-inline-start: auto""#));
+}