feat: custom front-layer shape in BackdropScaffold

[daadu]

• added frontLayerShape in-place of frontLayerBorderRadius
• deprecating frontLayerBorderRadius in back-compat way

Closes #115

[daadu]

tested with example app with following code:

BackdropScaffold(
    ...
    frontLayerShape: const BeveledRectangleBorder(
          borderRadius: BorderRadius.only(
            topLeft: Radius.circular(16),
            topRight: Radius.circular(16),
          ),
        ),
    ...
)