Simple sound effect map object

[RemieRichards]

• Adds a new step_trigger, "sound_effect", acts as a mapper friendly wrapper for playing sound effects
• Adds a new step_trigger variable, "mobs_only", like affects_ghosts this is a boolean for allowing/denying atom/movables from triggering step_triggers
• playsound_local() is now a /atom proc, all mob specific requirements have been moved to an override, by default playsound never calls playsound_local on non mobs, this is the usual behaviour, I did not make it call on all atoms in range as that would up the cost of playsound()

At request of WJ:

[WJohn]

I'd love to know to what extent I can toy with these things, and may end up bothering you later.
👍

[RemieRichards]

Alright @tgstation/commit-access Happy for a merge of this now, feel free to use your own judgement too.