feat(dartpad-preview): add markdown renderer #3935

[LeonMollNetlight]

Adds a markdown renderer and a preview/edit mode to the editor:

Fixes #3935

[github-actions]

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// Copyright (c) 2026, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
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pkgs/dart_services/lib/src/context.dart
pkgs/dartpad_preview/examples/counter/lib/main.dart
pkgs/dartpad_preview/examples/fibonacci/bin/main.dart
pkgs/dartpad_preview/examples/snippet_dart/bin/main.dart
pkgs/dartpad_preview/examples/sunflower/lib/main.dart
pkgs/samples/lib/brick_breaker.dart
pkgs/samples/lib/fibonacci.dart
pkgs/samples/lib/hello_world.dart
pkgs/samples/lib/main.dart
pkgs/samples/lib/sunflower.dart

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[dtetruash-nl]

Looked at the renderer. Gonna look at the codemirror and UI stuff later