UiImage helper functions (#8199)

# Objective

Add helper functions to `UiImage` for creating flipped images.

## Changelog

* Added `with_flip_x` and `with_flip_y` methods to `UiImage` that return
the `UiImage` flipped along the respective axis.

crates/bevy_ui/src/ui_node.rs

@@ -758,6 +758,20 @@ impl UiImage {
             ..Default::default()
         }
     }
+
+    /// flip the image along its x-axis
+    #[must_use]
+    pub const fn with_flip_x(mut self) -> Self {
+        self.flip_x = true;
+        self
+    }
+
+    /// flip the image along its y-axis
+    #[must_use]
+    pub const fn with_flip_y(mut self) -> Self {
+        self.flip_y = true;
+        self
+    }
 }
 
 impl From<Handle<Image>> for UiImage {