[WIP] Set up shared package for components and utilities.

[schultek]

Adds a new site_shared package that contains components and other utilities for sharing with dart-lang/site-www.

• Simply moved all components that are the same between both sites.
• Moved and adapted components that exist on both sites but are slightly different (e.g. SiteSwitcher, Quiz)
• Moved all styles related to a component that is also moved.
• Moved the markdown rendering setup, which is the same for both sites.
• Moved some utilities that are needed by components like analytics and code highlighting.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit be6ce85):

https://flutter-docs-prod--pr13259-feat-site-shared-r2bqgfbp.web.app

[gemini-code-assist]

Code Review

This pull request introduces a new site_shared package to centralize common components, utilities, and SCSS previously duplicated across the site and other packages. Key changes include extensive refactoring of SCSS imports, relocation of numerous Dart files to the new site_shared package with corresponding import path updates, and significant improvements to component handling and hydration logic, particularly for ComponentRef. Specific component updates include adding trailingIcon to Button, new parameters for CookieNotice and CopyButton, and refactoring quiz parsing in tutorial/quiz.dart. Review feedback points out two critical issues: inconsistent markdown parsing and weak validation in tutorial/quiz.dart for quizzes defined in tags, and a potential crash in util.dart's getOS() function due to web.window access during server-side rendering.

[gemini-code-assist]

The current implementation has two issues:

1. Inconsistent Parsing: Quizzes defined via tag content use Question.fromMap, while those loaded by ID use _parseQuestion. Since _parseQuestion explicitly handles markdown parsing via parseMarkdownToHtml, quizzes defined directly in tags will likely fail to render markdown correctly (showing raw markdown text instead of HTML).
2. Weak Validation: Using assert for input validation is insufficient for production builds where assertions are disabled. If the YAML structure is incorrect, the code will throw a TypeError during the cast to YamlList instead of providing a descriptive error message.

    final data = loadYamlNode(content);
    if (data is! YamlList) {
      throw ArgumentError('Invalid Quiz content. Expected a YAML list.');
    }
    final questions = data.nodes
        .map((n) => _parseQuestion(n as YamlMap))
        .toList();
    if (questions.isEmpty) {
      throw ArgumentError('Quiz must contain at least one question.');
    }

[gemini-code-assist]

The getOS() function accesses web.window, which is not available during server-side rendering (SSR) and will cause a crash. Since this is a shared utility, it should be environment-aware. Note: You will need to import package:jaspr/jaspr.dart to use kIsWeb.

OperatingSystem? getOS() {
  if (!kIsWeb) return null;
  final userAgent = web.window.navigator.userAgent;

[sfshaza2]

lgtm. I leave it to you, @schultek, to fix the build issues.

[parlough]

Thanks for tackling this Kilian! I'm excited.

Hold off on landing this. I'd like to take a look first. I should have time early next week.