Set up shared package for components and utilities (#13486)

Adds a new `site_shared` package that contains components and other
utilities for sharing with
[dart-lang/site-www](https://github.com/dart-lang/site-www).

- Moved all components that are the same between both sites.
- Moved and adapted components that exist on both sites but are slightly
different (such as `SiteSwitcher` and `Quiz`)
- Moved styled used by moved components.
- Moved the markdown rendering setup, which is the same for both sites.
- Moved some utilities that are needed by extracted components, such as
analytics and code highlighting.

---------

Co-authored-by: Kilian Schulte <netlight@schultek.dev>

Author: parlough

packages/site_shared/README.md

@@ -0,0 +1,48 @@
+# site_shared
+
+This package is the core library containing
+shared logic, UI components, and the design system for
+the Dart and Flutter documentation sites.
+
+It provides a centralized location for APIs,
+user interface elements, and logic intended for use by
+both the `dart.dev` and `docs.flutter.dev` websites.
+Using a shared package ensures a consistent design language and
+feature set across Dart and Flutter web documentation platforms.
+
+## What's included
+
+The `site_shared` package provides several key capabilities to
+build documentation websites using Dart, Jaspr, and Jaspr Content:
+
+- **UI components** (`lib/components`):
+  Reusable, modular components built for use across documentation pages.
+  - **Common components** (`lib/components/common`):
+    Everyday UI elements such as breadcrumbs, buttons, code blocks, and more.
+  - **Layout components** (`lib/components/layout`):
+    Structural layout elements like theme switchers,
+    site switchers, banners, and menu toggles.
+  - **Interactive components**:
+    Integrations such as Dartpad (`lib/components/dartpad`),
+    tutorials, and user client-side feedback tools.
+- **Markdown extensions and processors** (`lib/extensions`):
+  Custom processors that hook into the Dart Markdown parser to
+  extend its default syntax and behavior, such as `attribute_processor.dart`.
+- **Core styles** (`lib/_sass`):
+  The shared base styles and component-specific SCSS styling.
+  These resources define the unified visual identity used by both websites.
+- **Utilities and builders** (`lib/src`):
+  Reusable logic for code syntax highlighting (`lib/src/highlight`),
+  analytics integrations (`lib/src/analytics`),
+  builders (`lib/src/builders`), and various helper utilities.
+
+## Goals
+
+The primary aims of this shared package are to:
+
+1.  Streamline styling and standardize UI component implementation
+    across our various websites.
+1.  Prevent code duplication between the
+    `dart-lang/site-www` and `flutter/website` repositories.
+1.  Establish a robust codebase that can be
+    updated, maintained, and improved in a unified way.

packages/site_shared/analysis_options.yaml

@@ -0,0 +1,4 @@
+include: package:analysis_defaults/analysis.yaml
+
+formatter:
+  trailing_commas: preserve

packages/site_shared/build.yaml

@@ -0,0 +1,14 @@
+builders:
+  stylesHashBuilder:
+    import: "package:site_shared/src/builders/styles_hash_builder.dart"
+    builder_factories: ["stylesHashBuilder"]
+    build_extensions:
+      "web/assets/css/main.css":
+        - "lib/src/style_hash.dart"
+    auto_apply: dependents
+    build_to: source
+    required_inputs:
+      - ".css"
+    defaults:
+      dev_options:
+        fixed_hash: true

sites/docs/lib/_sass/base/_mixins.scss …/site_shared/lib/_sass/base/_mixins.scsssites/docs/lib/_sass/base/_mixins.scss renamed to packages/site_shared/lib/_sass/base/_mixins.scss sites/docs/lib/_sass/base/_mixins.scss renamed to packages/site_shared/lib/_sass/base/_mixins.scss

@@ -25,7 +25,8 @@ button {
   cursor: pointer;
 
   &.filled-button,
-  &.text-button, &.outlined-button {
+  &.text-button,
+  &.outlined-button {
     display: flex;
     align-items: center;
     width: fit-content;

sites/docs/lib/_sass/base/_reset.scss …s/site_shared/lib/_sass/base/_reset.scsssites/docs/lib/_sass/base/_reset.scss renamed to packages/site_shared/lib/_sass/base/_reset.scss sites/docs/lib/_sass/base/_reset.scss renamed to packages/site_shared/lib/_sass/base/_reset.scss

@@ -173,7 +173,7 @@
     }
   }
 
-    &.install-card {
+  &.install-card {
     gap: 0.25rem;
 
     .card-leading {