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 c4007b2):

https://flutter-docs-prod--pr13259-feat-site-shared-oeuo1lvo.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.

[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.

[parlough]

Exciting!! I haven't tested the stage site yet, but generally the direction looks great to me.

I had a few small comments and main has some updates to integrate.

Thanks so much for tackling this :D

[parlough]

Can we move the individual syntaxes to the src directory?

[parlough]

Can you add a basic README to this directory as well stating something about how it's intended for APIs meant to be used by dart.dev and docs.flutter.dev.

[schultek]

generated something