✨ (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/html_props_flex_item.rs

@@ -0,0 +1,119 @@
+use pagetop::prelude::*;
+
+#[pagetop::test]
+async fn default_flex_item_adds_nothing() {
+    let props = Props::default().with_prop(PropsOp::flex_item(FlexItem::new()));
+
+    assert_eq!(props.get_classes(), None);
+    assert_eq!(props.get_styles(), None);
+}
+
+#[pagetop::test]
+async fn grow_adds_flex_grow_style() {
+    let props = Props::default().with_prop(PropsOp::flex_item(
+        FlexItem::new().with_grow(flex::ItemGrow::Is1),
+    ));
+
+    assert_eq!(props.get_styles(), Some("flex-grow: 1".to_string()));
+}
+
+#[pagetop::test]
+async fn shrink_adds_flex_shrink_style() {
+    let props = Props::default().with_prop(PropsOp::flex_item(
+        FlexItem::new().with_shrink(flex::ItemShrink::Is0),
+    ));
+
+    assert_eq!(props.get_styles(), Some("flex-shrink: 0".to_string()));
+}
+
+#[pagetop::test]
+async fn align_self_adds_matching_style() {
+    let props = Props::default().with_prop(PropsOp::flex_item(
+        FlexItem::new().with_align_self(flex::ItemAlign::Center),
+    ));
+
+    assert_eq!(props.get_styles(), Some("align-self: center".to_string()));
+}
+
+#[pagetop::test]
+async fn order_adds_matching_style() {
+    let props = Props::default().with_prop(PropsOp::flex_item(
+        FlexItem::new().with_order(flex::ItemOrder::First),
+    ));
+
+    assert_eq!(props.get_styles(), Some("order: -129".to_string()));
+}
+
+#[pagetop::test]
+async fn size_percent_adds_flex_basis_style() {
+    let props = Props::default().with_prop(PropsOp::flex_item(
+        FlexItem::new().with_size(flex::ItemSize::Percent33),
+    ));
+
+    assert_eq!(props.get_styles(), Some("flex-basis: 33.3333%".to_string()));
+}
+
+#[pagetop::test]
+async fn size_custom_adds_flex_basis_style() {
+    let props = Props::default().with_prop(PropsOp::flex_item(
+        FlexItem::new().with_size(flex::ItemSize::Custom(UnitValue::Zero)),
+    ));
+
+    assert_eq!(props.get_classes(), None);
+    assert_eq!(props.get_styles(), Some("flex-basis: 0".to_string()));
+}
+
+#[pagetop::test]
+async fn offset_percent_adds_margin_inline_start_style() {
+    let props = Props::default().with_prop(PropsOp::flex_item(
+        FlexItem::new().with_offset(flex::ItemOffset::Percent33),
+    ));
+
+    assert_eq!(
+        props.get_styles(),
+        Some("margin-inline-start: 33.3333%".to_string())
+    );
+}
+
+#[pagetop::test]
+async fn offset_custom_adds_margin_inline_start_style() {
+    let props = Props::default().with_prop(PropsOp::flex_item(
+        FlexItem::new().with_offset(flex::ItemOffset::Custom(UnitValue::Px(16))),
+    ));
+
+    assert_eq!(
+        props.get_styles(),
+        Some("margin-inline-start: 16px".to_string())
+    );
+}
+
+#[pagetop::test]
+async fn combines_several_facets_in_one_call() {
+    let props = Props::default().with_prop(PropsOp::flex_item(
+        FlexItem::new()
+            .with_grow(flex::ItemGrow::Is1)
+            .with_shrink(flex::ItemShrink::Is0)
+            .with_align_self(flex::ItemAlign::Start)
+            .with_order(flex::ItemOrder::Is2)
+            .with_size(flex::ItemSize::Custom(UnitValue::Zero))
+            .with_offset(flex::ItemOffset::Percent10),
+    ));
+
+    assert_eq!(
+        props.get_styles(),
+        Some(
+            "flex-grow: 1; flex-shrink: 0; align-self: flex-start; order: 2; flex-basis: 0; \
+             margin-inline-start: 10%"
+                .to_string()
+        )
+    );
+    assert_eq!(props.get_classes(), None);
+}
+
+#[pagetop::test]
+async fn from_flex_item_for_props_op() {
+    let item = FlexItem::new().with_grow(flex::ItemGrow::Is1);
+    let props = Props::default().with_prop(item.into());
+
+    assert_eq!(props.get_styles(), Some("flex-grow: 1".to_string()));
+}