Facilitating skipping custom alt-side intro

[SSM240]

Opening this issue on @l-Luna's request. My mod SSM's Quality of Life Mod has a feature to skip the B-side intro, but this doesn't currently work with the custom alt-side intro in this mod.

The easiest way to facilitate this would be to factor out this part in AltSidesHelperModule into a separate named coroutine, since this would make it fit perfectly into my own mod's functionality for skipping coroutines.

Luna also said it might want to add a skip button to this mod itself, though personally I feel it'd be a bit inconsistent if AltSidesHelper implemented its own separate skip behavior that only applied to the custom alt-side intro.