Make function Size::new const for bevy_ui widgets (#6602)

# Objective

Fixes #6594 

## Solution

- `New` function for `Size` is now a `const` function :)

## Changelog

- `New` function for `Size` is now a `const` function

## Migration Guide

- Nothing has been changed

Author: kerkmann

crates/bevy_ui/src/geometry.rs

@@ -338,7 +338,7 @@ impl Size {
     /// assert_eq!(size.width, Val::Px(100.0));
     /// assert_eq!(size.height, Val::Px(200.0));
     /// ```
-    pub fn new(width: Val, height: Val) -> Self {
+    pub const fn new(width: Val, height: Val) -> Self {
         Size { width, height }
     }