[Enhancement] Built in alt inst unlocking

[MidyGamy]

This add a built in check of available alt inst and if it's unlocked, allowing to remove that annoying function found in almost all song scripts (and will be easier do add more mixes)

  public override function listAltInstrumentalIds(difficultyId:String, variationId:String):Array<String> {
    if (difficultyId == 'easy' || difficultyId == 'normal' || difficultyId == 'hard') {
      var hasBeatenPicoMix = Save.instance.hasBeatenSong(this.id, null, 'pico');

      switch (variationId) {
        case 'pico':
          // return hasBeatenPicoMix ? [''] : [];
          // No Pico mix on BF instrumental, sorry!
          return [];
        default:
          return hasBeatenPicoMix ? ['pico'] : [];
      }
    }
  }

It also adds the unlockedInstByDefault property in the song metadata of a song alt to lock it (since it's unlocked by default if the property doesn't exits)

[MidyGamy]

Btw that also fixes a problem where a song with a Pico Mix couldn't have an other altInst using the _merge folder

[TechnikTil]

i made something like this on FunkinCrew/funkin.assets#105

[MidyGamy]

i made something like this on FunkinCrew/funkin.assets#105

Yeah, but you have to manually put it every times, here it's built in, with just the need of a check box in the chart editor

[TechnikTil]

i made something like this on FunkinCrew/funkin.assets#105

Yeah, but you have to manually put it every times, here it's built in, with just the need of a check box in the chart editor

o ok

[AbnormalPoof]

This PR appears to have merge conflicts. Please fix them!