Use Web Audio for browser voice meter

lib/audio/browser_voice_meter_stub.dart

@@ -0,0 +1,16 @@
+class VoiceMeterFrame {
+  const VoiceMeterFrame({required this.level, required this.samples});
+
+  final double level;
+  final List<double> samples;
+}
+
+class BrowserVoiceMeter {
+  bool get isSupported => false;
+
+  Future<void> start(void Function(VoiceMeterFrame frame) onFrame) async {}
+
+  Future<void> stop() async {}
+
+  void dispose() {}
+}