LibURL: Promote Host to a proper class

This lets us move a few Host-related functions (like serialization and
checks for what the Host is) into Host instead of having them dotted
around the codebase.

For now, the interface is still very Variant-like, to avoid having to
change quite so much in one go.

Libraries/LibIPC/Decoder.cpp

@@ -111,6 +111,13 @@ ErrorOr<URL::Origin> decode(Decoder& decoder)
     return URL::Origin { move(scheme), move(host), port };
 }
 
+template<>
+ErrorOr<URL::Host> decode(Decoder& decoder)
+{
+    auto value = TRY(decoder.decode<URL::Host::VariantType>());
+    return URL::Host { move(value) };
+}
+
 template<>
 ErrorOr<File> decode(Decoder& decoder)
 {