Add map method to Components (#3210)

This PR adds a `map` method to the `Component` trait.

`map` is a fully-generalized form of `when`, as `when` can be expressed
in terms of `map`:

```rs
div().map(|this| if condition { then(this) } else { this })
```

This allows us to take advantage of Rust's pattern matching when
building up conditions:

```rs
// Before
div()
    .when(self.current_side == PanelSide::Left, |this| this.border_r())
    .when(self.current_side == PanelSide::Right, |this| {
        this.border_l()
    })
    .when(self.current_side == PanelSide::Bottom, |this| {
        this.border_b().w_full().h(current_size)
    })

// After
div()
    .map(|this| match self.current_side {
        PanelSide::Left => this.border_r(),
        PanelSide::Right => this.border_l(),
        PanelSide::Bottom => this.border_b().w_full().h(current_size),
    })
```

Release Notes:

- N/A

crates/ui2/src/elements/input.rs

@@ -94,14 +94,13 @@ impl Input {
             .active(|style| style.bg(input_active_bg))
             .flex()
             .items_center()
-            .child(
-                div()
-                    .flex()
-                    .items_center()
-                    .text_sm()
-                    .when(self.value.is_empty(), |this| this.child(placeholder_label))
-                    .when(!self.value.is_empty(), |this| this.child(label)),
-            )
+            .child(div().flex().items_center().text_sm().map(|this| {
+                if self.value.is_empty() {
+                    this.child(placeholder_label)
+                } else {
+                    this.child(label)
+                }
+            }))
     }
 }