feat: implement export as PDF (#14057)

I used [pdfmake](https://www.npmjs.com/package/pdfmake) to implement an
"export as PDF" feature, and I am happy to share with you!

This should fix #13577, fix #8846, and fix #13959.

A showcase:

[Getting
Started.pdf](https://github.com/user-attachments/files/24013057/Getting.Started.pdf)

Although it might miss rendering some properties currently, it can
evolve in the long run and provide a more native experience for the
users.


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

* **New Features**
- Experimental "Export to PDF" option added to the export menu (behind a
feature flag)
- PDF export supports headings, paragraphs, lists, code blocks, tables,
images, callouts, linked documents and embedded content

* **Chores**
  - Added PDF rendering library and consolidated PDF utilities
  - Feature flag introduced to control rollout

* **Tests**
  - Comprehensive unit tests added for PDF content rendering logic

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: DarkSky <darksky2048@gmail.com>

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

@@ -288,6 +288,15 @@ export const AFFINE_FLAGS = {
     configurable: isMobile,
     defaultState: false,
   },
+  enable_pdfmake_export: {
+    category: 'blocksuite',
+    bsFlag: 'enable_pdfmake_export',
+    displayName: 'Enable PDF Export',
+    description:
+      'Experimental export PDFs support, it may contain the wrong style.',
+    configurable: true,
+    defaultState: false,
+  },
 } satisfies { [key in string]: FlagInfo };
 
 // oxlint-disable-next-line no-redeclare