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/gpui2/src/element.rs

@@ -198,14 +198,19 @@ impl<V> AnyElement<V> {
 pub trait Component<V> {
     fn render(self) -> AnyElement<V>;
 
-    fn when(mut self, condition: bool, then: impl FnOnce(Self) -> Self) -> Self
+    fn map<U>(self, f: impl FnOnce(Self) -> U) -> U
+    where
+        Self: Sized,
+        U: Component<V>,
+    {
+        f(self)
+    }
+
+    fn when(self, condition: bool, then: impl FnOnce(Self) -> Self) -> Self
     where
         Self: Sized,
     {
-        if condition {
-            self = then(self);
-        }
-        self
+        self.map(|this| if condition { then(this) } else { this })
     }
 }