message: Fix default backgroundImageStretch to be `uniformToFill`
  This commit fixes the default value to comply with documentation.
  Closes
diff: @@ -96,7 +96,7 @@ namespace winrt::Microsoft::Terminal::Settings::Model::implementation
  GETSET_SETTING(hstring, BackgroundImagePath);
  GETSET_SETTING(double, BackgroundImageOpacity, 1.0);
  - GETSET_SETTING(Windows::UI::Xaml::Media::Stretch, BackgroundImageStretchMode, Windows::UI::Xaml::Media::Stretch::Fill);
  + GETSET_SETTING(Windows::UI::Xaml::Media::Stretch, BackgroundImageStretchMode, Windows::UI::Xaml::Media::Stretch::UniformToFill);
  GETSET_SETTING(Microsoft::Terminal::TerminalControl::TextAntialiasingMode, AntialiasingMode, Microsoft::Terminal::TerminalControl::TextAntialiasingMode::Grayscale);
  GETSET_SETTING(bool, RetroTerminalEffect, false);
  