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>

blocksuite/affine/shared/package.json

@@ -40,6 +40,7 @@
     "micromark-extension-gfm-task-list-item": "^2.1.0",
     "micromark-util-combine-extensions": "^2.0.0",
     "minimatch": "^10.1.1",
+    "pdfmake": "^0.2.20",
     "quick-lru": "^7.3.0",
     "rehype-parse": "^9.0.0",
     "rehype-stringify": "^10.0.0",
@@ -73,6 +74,7 @@
     "!dist/__tests__"
   ],
   "devDependencies": {
+    "@types/pdfmake": "^0.2.12",
     "vitest": "^3.2.4"
   },
   "version": "0.25.7"