Fix a warning for Ruby::UnannotatedEmptyCollection

```
lib/lrama/lexer/token/user_code.rb:19:23: [warning] Empty array doesn't have type annotation
│ Diagnostic ID: Ruby::UnannotatedEmptyCollection
│
└           references = []
                         ~~
```

lib/lrama/lexer/token/user_code.rb

@@ -16,20 +16,19 @@ def references
 
         def _references
           scanner = StringScanner.new(s_value)
-          references = []
 
-          until scanner.eos? do
-            case
-            when reference = scan_reference(scanner)
-              references << reference
-            when scanner.scan(/\/\*/)
-              scanner.scan_until(/\*\//)
-            else
-              scanner.getch
+          Enumerator.new do |y|
+            until scanner.eos?
+              case
+              when reference = scan_reference(scanner)
+                y << reference
+              when scanner.scan(/\/\*/)
+                scanner.scan_until(/\*\//)
+              else
+                scanner.getch
+              end
             end
-          end
-
-          references
+          end.to_a
         end
 
         def scan_reference(scanner)