\defaultfontfeatures[cascadiacode-extralight]
   {Extension=.otf,
    UprightFont=CascadiaCode-ExtraLight,
    ItalicFont=CascadiaCode-ExtraLightItalic,
    BoldFont=CascadiaCode-SemiLight,
    BoldItalicFont=CascadiaCode-SemiLightItalic,
    Contextuals=Alternate
}