Add modcharting features (using FunkinModchart) (#526)

Technically already in the codes but yeah

source/funkin/game/Note.hx

@@ -53,6 +53,13 @@ class Note extends FlxSprite
 	 */
 	public var nextSustain:Note;
 
+	/**
+	 * The parent of the sustain.
+	 * 
+	 * If this note is not sustain, this will be null.
+	 */
+	public var sustainParent:Null<Note>;
+
 	/**
 	 * Name of the splash.
 	 */
@@ -125,6 +132,10 @@ class Note extends FlxSprite
 		for(field in Reflect.fields(noteData)) if(!DEFAULT_FIELDS.contains(field))
 			this.extra.set(field, Reflect.field(noteData, field));
 
+		// work around to set the `sustainParent`
+		if (isSustainNote)
+			sustainParent = prevNote.isSustainNote ? prevNote.sustainParent : prevNote;
+
 		x += 50;
 		// MAKE SURE ITS DEFINITELY OFF SCREEN?
 		y -= 2000;

source/funkin/options/OptionsMenu.hx

@@ -39,6 +39,13 @@ class OptionsMenu extends TreeMenu {
 			state: LanguageOptions
 		},
 		#end
+		#if MODCHARTING_FEATURES
+		{
+			name: 'Modchart Settings >',
+			desc: 'Customize your modcharting experience...',
+			state: ModchartingOptions
+		},
+		#end
 		{
 			name: 'optionsTree.miscellaneous-name',
 			desc: 'optionsTree.miscellaneous-desc',

source/funkin/options/categories/AppearanceOptions.hx

@@ -60,4 +60,4 @@ class AdvancedAppearanceOptions extends TreeMenuScreen {
 	private function __changeAntialiasing() {
 		FlxG.game.stage.quality = (FlxG.enableAntialiasing = Options.antialiasing) ? BEST : LOW;
 	}
-}
+}

source/funkin/options/categories/ModchartingOptions.hx

@@ -0,0 +1,17 @@
+package funkin.options.categories;
+
+#if MODCHARTING_FEATURES
+class ModchartingOptions extends TreeMenuScreen {
+	public override function new() {
+		super("Modcharting Options", "Customize your modcharting experience.");
+		add(new NumOption(
+			"Hold Subdivisions",
+			"Subdivides the arrow's hold/sustain tail for smoother visuals, higher values improve quality but can hurt performance",
+			1, // minimum
+			128, // maximum
+			1, // change
+			"modchartingHoldSubdivisions" // save name
+			)); // callback
+	}
+}
+#end