Add Environment concept to QML

It would be nice if QML had a dedicated type for specifying "environments" that are inherited by child items:

App.qml

Item {
  Environment.theme: MyTheme{}

  ColumnLayout {
    StyledButton {}
    StyledButton {}
  }
}

StyledButton.qml

Item {
  Rectangle { color: Environment.theme.buttonBackgroundColor }
  ...
}

This is similar to what QQuickAttachedObject allows, but without having to use private api (QTBUG-63267) and without having to use C++.

Swift has a similar feature: https://developer.apple.com/documentation/swiftui/environment

A workaround for this today is context properties, but they have many downsides.