feat(editor): add experimental feature citation (#11984)

Closes: [BS-3122](https://linear.app/affine-design/issue/BS-3122/footnote-definition-adapter-适配)
Closes: [BS-3123](https://linear.app/affine-design/issue/BS-3123/几个-block-card-view-适配-footnote-态)

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

- **New Features**
  - Introduced a new citation card component and web element for displaying citations.
  - Added support for citation-style rendering in attachment, bookmark, and linked document blocks.
  - Enabled citation parsing from footnote definitions in markdown for attachments, bookmarks, and linked docs.
  - Added a feature flag to enable or disable citation features.
  - Provided new toolbar logic to disable downloads for citation-style attachments.

- **Improvements**
  - Updated block models and properties to support citation identifiers.
  - Added localization and settings for the citation experimental feature.
  - Enhanced markdown adapters to recognize and process citation footnotes.
  - Included new constants and styles for citation card display.

- **Bug Fixes**
  - Ensured readonly state is respected in block interactions and rendering for citation blocks.

- **Documentation**
  - Added exports and effects for new citation components and features.

- **Tests**
  - Updated snapshots to include citation-related properties in block data.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/core/src/modules/feature-flag/constant.ts

@@ -105,6 +105,16 @@ export const AFFINE_FLAGS = {
     configurable: isCanaryBuild,
     defaultState: isCanaryBuild,
   },
+  enable_citation: {
+    category: 'blocksuite',
+    bsFlag: 'enable_citation',
+    displayName:
+      'com.affine.settings.workspace.experimental-features.enable-citation.name',
+    description:
+      'com.affine.settings.workspace.experimental-features.enable-citation.description',
+    configurable: isCanaryBuild,
+    defaultState: isCanaryBuild,
+  },
 
   enable_emoji_folder_icon: {
     category: 'affine',