✨ (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_container.rs

@@ -0,0 +1,140 @@
+use pagetop::prelude::*;
+
+// **< Container >**********************************************************************************
+
+#[pagetop::test]
+async fn is_not_rendered_when_empty() {
+    let mut container = Container::new();
+    let html = container.render(&mut Context::default()).await;
+
+    assert!(html.is_empty());
+}
+
+#[pagetop::test]
+async fn default_kind_renders_a_div_element() {
+    let mut container = Container::new().with_child(Lc::n("x"));
+    let html = container
+        .render(&mut Context::default())
+        .await
+        .into_string();
+
+    assert!(html.starts_with("<div"));
+    assert!(html.ends_with("</div>"));
+}
+
+#[pagetop::test]
+async fn main_kind_renders_a_main_element() {
+    let mut container = Container::main().with_child(Lc::n("x"));
+    let html = container
+        .render(&mut Context::default())
+        .await
+        .into_string();
+
+    assert!(html.starts_with("<main"));
+    assert!(html.ends_with("</main>"));
+}
+
+// **< Container + Flex >***************************************************************************
+
+#[pagetop::test]
+async fn without_flex_no_style_attribute_is_added() {
+    let mut container = Container::new().with_child(Lc::n("x"));
+    let html = container
+        .render(&mut Context::default())
+        .await
+        .into_string();
+
+    assert!(!html.contains("style="));
+}
+
+#[pagetop::test]
+async fn default_flex_adds_only_display_flex() {
+    let mut container = Container::new()
+        .with_flex(Flex::row())
+        .with_child(Lc::n("x"));
+    let html = container
+        .render(&mut Context::default())
+        .await
+        .into_string();
+
+    assert!(html.contains(r#"style="display: flex""#));
+}
+
+#[pagetop::test]
+async fn column_direction_adds_flex_direction_style() {
+    let mut container = Container::new()
+        .with_flex(Flex::column())
+        .with_child(Lc::n("x"));
+    let html = container
+        .render(&mut Context::default())
+        .await
+        .into_string();
+
+    assert!(html.contains("display: flex"));
+    assert!(html.contains("flex-direction: column"));
+}
+
+#[pagetop::test]
+async fn wrap_justify_and_align_add_their_matching_styles() {
+    let mut container = Container::new()
+        .with_flex(
+            Flex::row()
+                .with_wrap(flex::Behavior::Wrap)
+                .with_justify(flex::ContentJustify::Center)
+                .with_align(flex::Align::Center)
+                .with_align_content(flex::AlignContent::SpaceBetween),
+        )
+        .with_child(Lc::n("x"));
+    let html = container
+        .render(&mut Context::default())
+        .await
+        .into_string();
+
+    assert!(html.contains("flex-wrap: wrap"));
+    assert!(html.contains("justify-content: center"));
+    assert!(html.contains("align-items: center"));
+    assert!(html.contains("align-content: space-between"));
+}
+
+#[pagetop::test]
+async fn gap_both_adds_a_single_gap_style() {
+    let mut container = Container::new()
+        .with_flex(Flex::row().with_gap(flex::Gap::Both(UnitValue::RelRem(0.5))))
+        .with_child(Lc::n("x"));
+    let html = container
+        .render(&mut Context::default())
+        .await
+        .into_string();
+
+    assert!(html.contains("gap: 0.5rem"));
+}
+
+#[pagetop::test]
+async fn gap_distinct_adds_row_and_column_gap_styles() {
+    let mut container = Container::new()
+        .with_flex(Flex::row().with_gap(flex::Gap::Distinct {
+            row: UnitValue::Px(4),
+            column: UnitValue::Px(8),
+        }))
+        .with_child(Lc::n("x"));
+    let html = container
+        .render(&mut Context::default())
+        .await
+        .into_string();
+
+    assert!(html.contains("row-gap: 4px"));
+    assert!(html.contains("column-gap: 8px"));
+}
+
+#[pagetop::test]
+async fn gap_none_adds_no_gap_style() {
+    let mut container = Container::new()
+        .with_flex(Flex::row())
+        .with_child(Lc::n("x"));
+    let html = container
+        .render(&mut Context::default())
+        .await
+        .into_string();
+
+    assert!(!html.contains("gap:"));
+}