Implement From<Val> for UiRect (#20312)

# Objective

Instead of:
```rust
Node {
    margin: UiRect::all(Val::Px(20.)),
    border: UiRect::all(Val::Px(5.)),
    padding: UiRect::all(Val::Px(10.)),
    ..default()
}
```
allow:
```rust
Node {
    margin: Val::Px(20.).into(),
    border: Val::Px(5.).into(),
    padding: Val::Px(10.).into(),
    ..default()
}
```

Which matches how single edge values are interpreted in CSS.

## Solution

Implement `From<Val>` for `UiRect`.

Author: ickshonpe

crates/bevy_ui/src/geometry.rs

@@ -687,6 +687,12 @@ impl Default for UiRect {
     }
 }
 
+impl From<Val> for UiRect {
+    fn from(value: Val) -> Self {
+        UiRect::all(value)
+    }
+}
+
 #[derive(Debug, Clone, Copy, PartialEq, Reflect)]
 #[reflect(Default, Debug, PartialEq)]
 #[cfg_attr(