Add axes_2d gizmo. (#12334)

# Objective

This PR addresses #12222 (Fixes #12222). Simple addition to add a 2D
axes gizmo.

## Solution

- Add a new method axes_2d which takes a transform and a case length and
then draws two arrows in the XY plane.

The only thing I'm not sure about here is taking a 3D transform as an
argument. It says in the transform comments that for 2D the z-axis is
used for ordering, so I figured I'd keep it that way?

---

## Changelog

- Add method axes_2d.
- Update arrow_2d to also calculate the tip length depending on arrow
length as in arrow.
- Add axes_2d to examples 2d_gizmos.

---------

Co-authored-by: Ben Lambert <[email protected]>

Author: lambertsbennett

crates/bevy_gizmos/src/arrows.rs

@@ -8,7 +8,7 @@ use bevy_color::{
     palettes::basic::{BLUE, GREEN, RED},
     Color,
 };
-use bevy_math::{Quat, Vec2, Vec3};
+use bevy_math::{Quat, Vec2, Vec3, Vec3Swizzles};
 use bevy_transform::TransformPoint;
 
 /// A builder returned by [`Gizmos::arrow`] and [`Gizmos::arrow_2d`]
@@ -201,4 +201,35 @@ where
         self.arrow(start, end_y, GREEN);
         self.arrow(start, end_z, BLUE);
     }
+
+    /// Draw a set of axes local to the given transform (`transform`), with length scaled by a factor
+    /// of `base_length`.
+    ///
+    /// This should be called for each frame the axes need to be rendered.
+    ///
+    /// # Example
+    /// ```
+    /// # use bevy_gizmos::prelude::*;
+    /// # use bevy_ecs::prelude::*;
+    /// # use bevy_transform::components::Transform;
+    /// # #[derive(Component)]
+    /// # struct AxesComponent;
+    /// fn draw_axes_2d(
+    ///     mut gizmos: Gizmos,
+    ///     query: Query<&Transform, With<AxesComponent>>,
+    /// ) {
+    ///     for &transform in &query {
+    ///         gizmos.axes_2d(transform, 1.);
+    ///     }
+    /// }
+    /// # bevy_ecs::system::assert_is_system(draw_axes_2d);
+    /// ```
+    pub fn axes_2d(&mut self, transform: impl TransformPoint, base_length: f32) {
+        let start = transform.transform_point(Vec3::ZERO);
+        let end_x = transform.transform_point(base_length * Vec3::X);
+        let end_y = transform.transform_point(base_length * Vec3::Y);
+
+        self.arrow_2d(start.xy(), end_x.xy(), RED);
+        self.arrow_2d(start.xy(), end_y.xy(), GREEN);
+    }
 }