Assignable Sounds to Countdown (per-stage)

[crowplexus]

these changes allow for custom intro sounds based on stage files
also adds an additional image when the narrator says "3" during the countdown, which can be toggled by adding an image called prepare into the images folder in any mod (meaning it isn't enabled by default)

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supersedes #15434 and fixes #14474

• Supersedes the aforementioned PR as you can simply put an empty string, null, or an empty array in the intro sounds array, making it way more customisable

[LarryFrosty]

rip goofy null trace

[TheOfficialSwords]

this is Fire.

[PartnerCapPikachu]

ily

[DMMaster636]

really nice! def makes some stuff easier
i have a few more ideas for improvements if this does get merged

[crowplexus]

I might make a countdown class to make this process less cluttered
spoiler: I didn't