add voices value returned by the backends

src/backends/speech_dispatcher.rs

@@ -75,6 +75,7 @@ impl Backend for SpeechDispatcher {
             pitch: true,
             volume: true,
             is_speaking: true,
+            voices: false,
             utterance_callbacks: true,
         }
     }

src/backends/web.rs

@@ -47,6 +47,7 @@ impl Backend for Web {
             pitch: true,
             volume: true,
             is_speaking: true,
+            voices: true,
             utterance_callbacks: true,
         }
     }

src/backends/winrt.rs

@@ -140,6 +140,7 @@ impl Backend for WinRT {
             pitch: true,
             volume: true,
             is_speaking: true,
+            voices: true,
             utterance_callbacks: true,
         }
     }