Include fallbacks into CSS properties of dynamic viewport units (svh, dvh, lvh)

[toniopelo]

This would be great to have an included fallback to old vh units when using new dynamic ones, in case the device doesn't support it.
Now we have to create our own overrides of h-{s,d,l}vh to have a proper fallback.
On older phones (ios14 or Opera mobile and so on) using h-{s,d,l}vh result in no height being set at all.
I think we can assume that the intent behind using h-{s,d,l}vh (100{s,d,l}vh) can translate to h-screen (100vh) on unsupported devices.