[BUGFIX] Fix the song preview restarting when changing difficulties within the same variation

[VioletSnowLeopard]

Does this PR close any issues? If so, link them below.

Fixes https://github.com/FunkinCrew/Funkin/issues/4821

Reopening of #4832

Briefly describe the issue(s) fixed.

The recent update changed the calls to generateSongList after difficulty changes to set onlyIfChanged to false. This would make them do their switching animation, but it would also call changeSelection which resets the song preview.

This PR fixes this issue by adding a parameter to changeSelection with a default value of true. If this value is false, playCurSongPreview won't get called. I set the call to changeSelection in generateSongList to pass in a value of false over the default true. Because changeSelection is called in several different places, this change doesn't cause the song preview to not restart when it should.

I also had to fix changeDiff to ensure its call to playCurSongPreview worked. When switching to erect difficulty on a song that doesn't have it, it will change the selected song to the closest song which does have it. I added in an update to characterVariations for the new selected song so currentVariation would be updated correctly.

Include any relevant screenshots or videos.

https://github.com/user-attachments/assets/77e642ff-837d-4804-a721-300a4a6af384

[Hundrec]

Good change, though it seems a similar change was made internally. I'll leave this open until we know for sure

[Hundrec]

Song previews function properly now, so I will close this. Thank you for contributing!