LibJS: Use HostImportModuleDynamically in ShadowRealmImportValue

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.

Userland/Libraries/LibJS/Tests/builtins/ShadowRealm/async-module.mjs

@@ -0,0 +1,11 @@
+await Promise.resolve(0);
+
+export const foo = "Well hello async shadows";
+
+await 1;
+
+export default "Default export";
+
+await Promise.resolve(2);
+
+export const bar = "'bar' export";