Add lsvoices command

src/tts/BaseEngine.ts

@@ -37,6 +37,16 @@ export abstract class BaseEngine {
     return this.voices[voice] != null;
   }
 
+  /** Returns sorted lowercase voice names known to this engine. Empty if the engine accepts any voice. */
+  listVoices(): string[] {
+    return Object.keys(this.voices).sort();
+  }
+
+  /** True when this engine has no static voice table and accepts any voice string. */
+  isFreeformVoice(): boolean {
+    return Object.keys(this.voices).length === 0;
+  }
+
   /** Default implementation: subclass should override either this or getSpeechFile. */
   async getSpeech(_text: string, _voice?: string, _params?: VoiceParams): Promise<Response> {
     throw new Error(`${this.shortName}: getSpeech not implemented`);