Now that module loading is implemented this just works :^).
Since ShadowRealm explicitly passed a null ScriptOrModule we attempt to
get the top most ScriptOrModule in HostImportModuleDynamically.
This won't work in general as the web specifies other behavior but for
LibJS there must always be an active script to call
HostImportModuleDynamically.
Well... sort of. This adds the function itself and all the scaffolding
from the ShadowRealm API (and basically completes its implementation).
However, we do not nearly have enough support for modules and imports,
so we currently pretend whatever was attempted to be imported failed -
once we have HostImportModuleDynamically it should be relatively easy to
complete the implementation.
