Add ability to set custom default alternate animation suffix (#882)

The alt. animation toggle event currently does not support setting the
suffix directly,
but that's outside of the scope of adding this feature at the moment.

source/funkin/backend/system/Flags.hx

@@ -163,6 +163,8 @@ class Flags {
 	// Font configuration
 	public static var DEFAULT_FONT:String = "vcr.ttf";
 	public static var DEFAULT_FONT_SIZE:Int = 16;
+	
+	public static var DEFAULT_ALT_ANIM_SUFFIX:String = "-alt";
 
 	// to translate these you need to convert them into ids
 	// Resume -> pause.resume

source/funkin/game/PlayState.hx

@@ -1667,7 +1667,7 @@ class PlayState extends MusicBeatState
 					if (strLine.characters != null) // Alt anim Idle
 						for (character in strLine.characters) {
 							if (character == null) continue;
-							character.idleSuffix = event.params[1] ? "-alt" : "";
+							character.idleSuffix = event.params[1] ? strLine.defaultAnimSuffix : "";
 						}
 				}
 			case "Play Animation":

source/funkin/game/StrumLine.hx

@@ -74,6 +74,10 @@ class StrumLine extends FlxTypedGroup<Strum> {
 	 * Which animation suffix on characters that should be used when hitting notes.
 	 */
 	public var animSuffix(default, set):String = "";
+	/**
+	 * The current animation suffix the strumline should use. Note that setting this will only take effect upon the alt. animation being reset.
+	 */
+	public var defaultAnimSuffix:String = Flags.DEFAULT_ALT_ANIM_SUFFIX;
 	/**
 	 * TODO: Write documentation about this being a variable that can help when making multi key
 	 */
@@ -468,11 +472,11 @@ class StrumLine extends FlxTypedGroup<Strum> {
 		return animSuffix = str;
 	}
 	private inline function set_altAnim(b:Bool):Bool {
-		animSuffix = b ? "-alt" : "";
+		animSuffix = b ? defaultAnimSuffix : "";
 		return b;
 	}
 	private inline function get_altAnim():Bool {
-		return animSuffix == "-alt";
+		return animSuffix == defaultAnimSuffix;
 	}
 	#end
 }