Add "silent" audio track option to exports for App Store compatibility

App Store Preview videos require audio tracks even if there is no audio. A popular workaround is to add a silent audio track separately in e.g. iMovie but it would be great if Screen Studio just did that directly.

https://www.reddit.com/r/iOSProgramming/comments/rj3mfa/your_app_preview_contains_unsupported_or/?rdt=44883