Added LocalChatsound (localcs) action

[bravelycowering]

Similarly to localmsg, localcs plays a chatsound for everyone in the level. While this can technically be achieved to some degree using localmsg, those without the plugin would receive the chatsounds as normal chat messages, and there's no easy way to play the sound from a specific bot or player.

Syntax is identical to the cs action, with the exception of a new all subcommand that plays the sound to everyone in the level similarly to how insounds/outsounds play.

I did also slightly change how HasPlugin._HasPlugin works internally in order to do this cleanly, since the script needs to check for the chatsounds plugin for each player the sound is playing for, rather than just the script runner.