Support mask.strong prop for increased mask opacity

[amcclain]

Off the back of #2690, which made load masks more transparent by default, we sometimes have a use case for masking a panel with a stronger, more opaque mask - e.g. when the overall layout of the masked and surrounding components don't make it sufficiently clear where the borders of the mask are, or for any other reason.

Proposed solution is to support a strong prop on mask. Could add a --xh-mask-strong-bg CSS var to support, defaulting to --xh-backdrop-bg.

Should be able to configure into panel masking like:

panel({
  mask: {observer: model.loadModel, strong: true}
})