[Could Have] Add custom swap sounds level-by-level

[Morxemplum]

In 1.92, custom swap systems had a creative outlet by allowing pack developers to make their own sound with the swap. This technically is possible with 2.x's onInput function, but that means you wouldn't be able to have the "Swap Enabled" on the top left and there would be no color to enable for swapping (unless you do it manually with style switching). Perhaps there could be a lua function that can be added to override the programmer swap sound with a custom one in the "Sounds" folder.

[Morxemplum]

This is definitely something that can easily be done with the new a_ prefix that just recently got added. I can have this issue solved as part of a PR expanding that prefix.