Add a way to make `Spring.PlaySoundFile` obey LoS

Add some way to make it so that position-based `Spring.PlaySoundFile` only plays if the local player has LoS over given position (or is a spectator with `specFullView`).

This can be a new boolean param to the existing function (defaulting to ignoring LoS for backward compatibility), though the param list there is pretty confusing already and the param would be meaningless without a specified position. So likely it would be better to have a new function where the position parameters are mandatory too.

Start by locating the implementation and API declaration for Spring.PlaySoundFile, then inspect how positional sounds and line-of-sight or spectator visibility are handled. Define the new or extended entry point so positional playback checks the requested visibility condition while preserving existing behavior; done means the API is implemented and its behavior is covered by the relevant engine tests.

Written by the indexing model from the issue text.