feat: add type alias support

[klaidliadon]

Adds Dart typedef rendering for RIDL type aliases.

Changes

• types.go.tmpl: add isAliasType check between enum and struct blocks, emitting typedef Name = BaseType;
• _examples/flutter-go/service.ridl: add ItemName and Quantity alias definitions
• go.mod / go.sum: add github.com/webrpc/webrpc@v0.36.1 dependency
• Regenerated example output in flutter_app/lib/generated/sdk.dart

Generated output

typedef ItemName = String;
typedef Quantity = int;