message: Fix 'background_opacity' deprecation
  During the deprecation of the 'background_opacity' field, it was
  incorrectly renamed to 'window_opacity'. This changes that back to ensure
  the old field still works and a warning is emitted accordingly.
  See the original regression here:
  Fixes
diff: @@ -70,7 +70,7 @@ pub struct UiConfig {
  /// Background opacity from 0.0 to 1.0.
  #[config(deprecated = "use window.opacity instead")]
  - window_opacity: Option<Percentage>,
  + background_opacity: Option<Percentage>,
  }
  impl Default for UiConfig {
  @@ -85,7 +85,7 @@ impl Default for UiConfig {
  config_paths: Default::default(),
  key_bindings: Default::default(),
  mouse_bindings: Default::default(),
  - window_opacity: Default::default(),
  + background_opacity: Default::default(),
  bell: Default::default(),
  colors: Default::default(),
  draw_bold_text_with_bright_colors: Default::default(),
  @@ -117,7 +117,7 @@ impl UiConfig {
  #[inline]
  pub fn window_opacity(&self) -> f32 {
  - self.window_opacity.unwrap_or(self.window.opacity).as_f32()
  + self.background_opacity.unwrap_or(self.window.opacity).as_f32()
  }
  #[inline]
  