add beans_4_0.xsd for CDI 4.0

[Ladicek]

For CDI 4.0, we have to add beans_4_0.xsd. I'm filing this issue to highlight that we should adjust the XSD documentation to point out that in CDI Lite environments, only bean-discovery-mode is observed (and all is not a valid value); everything else is ignored.

The XSD documentation also doesn't mention anything about empty beans.xml. If we go with making annotated the default in case of empty beans.xml, maybe we should mention that in the XSD?