FlxPartialSound does not work properly in iOS

[moondroidcoder]

This pull-request #2 fixes polymod implementation for FlxPartialSound, it also is supposed to fix mobile implementation.
The (now reverted) commit c3dffed is supposed to fix issues with iOS platforms not being able to load freeplay audio.

But because it was reverted, there's an issue that popped up. iOS Native can't play base-game audio anymore on funkin, but mods (zipped or not) work with no problems.

iOS Footage for the base game:

RPReplay_Final1718977303.mov

iOS Footage for base game w/ mods:

RPReplay_Final1718986078.mov

Since iOS is really similar to Android, i think it's worth to mention that android functions flawlessly with the current implementation. Mods, zip mods, and base game audio were all loaded and played correctly.

Android Footage:

screen-20240621-171324.mp4

I think this is a fairly easy fix since unreverting the commit practically fixes everything! We tested it with our own fork.