Add debug rending to highlight UI features during development.

[Andrious]

Supplying Flutter's rendering library will further assist developers when working on an app's interface:

import 'package:flutter/rendering.dart' as debug
    show
        debugPaintSizeEnabled,
        debugPaintBaselinesEnabled,
        debugPaintPointersEnabled,
        debugPaintLayerBordersEnabled,
        debugRepaintRainbowEnabled,
        debugRepaintTextRainbowEnabled;

Assigning true to any of the following will highlight particular features of the interface:

/// Causes each RenderBox to paint a box around its bounds debugPaintSizeEnabled

/// Causes each RenderBox to paint a line at each of its baselines. bool debugPaintBaselinesEnabled

/// Causes each Layer to paint a box around its bounds. bool debugPaintLayerBordersEnabled

/// Causes objects like [RenderPointerListener] to flash while they are being tapped. bool debugPaintPointersEnabled

/// Overlay a rotating set of colors when repainting layers in debug mode. bool debugRepaintRainbowEnabled

/// Overlay a rotating set of colors when repainting text in debug mode. bool debugRepaintTextRainbowEnabled

For example, with it now implemented, the debugPaintSizeEnabled parameter set true will visually indicate the size and boundaries of the separate widgets that make up a particular screen:

Pre-launch Checklist

• [^] I have updated CHANGELOG.md with my changes
• [^] I have run "dart format ." on the project
• [ ] I have added/updated relevant documentation

[bdlukaa]

I am not sure this change should be proposed to fluent_ui. Flutter's WidgetsApp doesn't have this built-in, why would FluentApp do? Also, these can be easily toggled using the command line or Dart DevTools.

[Andrious]

Very well. Excellent work, D'Luka. I wish you continued success.