Create `PaneItemWidget`

[md-rifatkhan]

Custom Navigation Pane Widget
It will better to support custom navigation pane widget.

Because sometime we need to add slider or custom button on navigation view

[bdlukaa]

You can create a PaneItem, just as you would create a widget.

fluent_ui/example/lib/main.dart

Lines 665 to 701 in 0b71276

	class _LinkPaneItemAction extends PaneItem {
	_LinkPaneItemAction({
	required super.icon,
	required this.link,
	required super.body,
	super.title,
	});
	final String link;
	@override
	Widget build(
	BuildContext context,
	bool selected,
	VoidCallback? onPressed, {
	PaneDisplayMode? displayMode,
	bool showTextOnTop = true,
	bool? autofocus,
	int? itemIndex,
	}) {
	return Link(
	uri: Uri.parse(link),
	builder: (context, followLink) => Semantics(
	link: true,
	child: super.build(
	context,
	selected,
	followLink,
	displayMode: displayMode,
	showTextOnTop: showTextOnTop,
	itemIndex: itemIndex,
	autofocus: autofocus,
	),
	),
	);
	}
	}

Note that it behaves as a pane item, and it has an index.

The creation of a PaneItemWidget would be a good addition.