feat(schema): add ReferenceType interface

schema/enum.go

@@ -41,7 +41,8 @@ func (e EnumType) TypeName() string {
 	return e.Name
 }
 
-func (EnumType) isType() {}
+func (EnumType) isType()          {}
+func (EnumType) isReferenceType() {}
 
 // Validate validates the enum definition.
 func (e EnumType) Validate(TypeSet) error {

schema/type.go

@@ -13,6 +13,15 @@ type Type interface {
 	isType()
 }
 
+// ReferenceType is a marker interface that all types that can be the target of Field.ReferencedType implement.
+// Currently, this is only EnumType.
+type ReferenceType interface {
+	Type
+
+	// IsReferenceType is implemented if this is a reference type.
+	isReferenceType()
+}
+
 // TypeSet represents something that has types and allows them to be looked up by name.
 // Currently, the only implementation is ModuleSchema.
 type TypeSet interface {