Implement From<Val> for Fontsize (#24076)

# Objective

Implement `From<Val>` for `Fontsize` so that the `Val` `px`, `vh`, etc
helpers to be used to set font sizes in `bsn! `.

## Solution

Implement `From<Val>` for `Fontsize`, with `Val::Auto` mapped to
`FontSize:Rem(1.)` and `Val::Percent(x)` mapped to
`FontSize::Rem(x/100.)`.

Author: ickshonpe

crates/bevy_ui/src/geometry.rs

@@ -1,5 +1,6 @@
 use bevy_math::{MismatchedUnitsError, StableInterpolate as _, TryStableInterpolate, Vec2};
 use bevy_reflect::{std_traits::ReflectDefault, Reflect};
+use bevy_text::FontSize;
 use bevy_utils::default;
 use core::ops::{Div, DivAssign, Mul, MulAssign, Neg};
 use thiserror::Error;
@@ -468,6 +469,20 @@ impl Val {
     }
 }
 
+impl From<Val> for FontSize {
+    fn from(value: Val) -> Self {
+        match value {
+            Val::Auto => FontSize::Rem(1.),
+            Val::Px(px) => FontSize::Px(px),
+            Val::Percent(percent) => FontSize::Rem(percent / 100.),
+            Val::Vw(vw) => FontSize::Vw(vw),
+            Val::Vh(vh) => FontSize::Vh(vh),
+            Val::VMin(vmin) => FontSize::VMin(vmin),
+            Val::VMax(vmax) => FontSize::VMax(vmax),
+        }
+    }
+}
+
 impl TryStableInterpolate for Val {
     type Error = MismatchedUnitsError;
 

examples/scene/bsn.rs

@@ -52,7 +52,7 @@ fn button(label: &str) -> impl Scene {
             Text(label)
             TextFont {
                 font: FontSourceTemplate::Handle("fonts/FiraSans-Bold.ttf"),
-                font_size: FontSize::Px(33.0),
+                font_size: px(33.0),
             }
             TextColor(Color::srgb(0.9, 0.9, 0.9))
             TextShadow