fix(bluetooth-serial): add missing disconnect method

closes #1151

src/plugins/bluetoothserial.ts

@@ -60,6 +60,13 @@ export class BluetoothSerial {
   })
   static connectInsecure(macAddress: string): Observable<any> { return; }
 
+  /**
+   * Disconnect from the connected device
+   * @returns {Promise<any>}
+   */
+  @Cordova()
+  static disconnect(): Promise<any> { return; }
+
   /**
    * Writes data to the serial port
    * @param {any} data ArrayBuffer of data