Adjust build script and properties for use with dual audio releases

[MarioLuigi0404]

Possibly not the most elegant approach, but adds functions for merging S&S tracks, configures mux function and sub properties file for two audio tracks and S&S sub track.

Of note, muxing the S&S track requires opsync/edsync lines to be in files other than dialogue.

(Not strictly tested but made based on a separate multi-audio project repo that I know does work. If things are broken, yell at me.)

[petzku]

Of note, muxing the S&S track requires opsync/edsync lines to be in files other than dialogue.

it may be cleaner to simply strip the dialogue lines from these tracks, rather than merge them separately. this also avoids duplicating the merge logic:

fun EventLine.isDialogue(): Boolean {
    return Regex("Default|Alt") in this.style
}
subs {
// -- snip --
    // Build signs & songs track
    val forced by task<ASS> {
        from(cleanmerge.item())
        ass {
            events.lines.removeIf { it.isDialogue() }
        }
    }

[MarioLuigi0404]

Fixes issue with S&S requiring sync points in a non-dialogue file, though also purges any potential foreign dialogue that should be in the S&S track unless it is given a style name that doesn't get picked up by the dialogue regex. I don't think it's much of an issue though, since handling for that wasn't really implemented to begin with.

[petzku]

If that was desired, we could merge in a Forced.ass later on in the chain. Definitely worth thinking about.