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/widgets/linked-doc/src/transformers/index.ts

@@ -2,5 +2,6 @@ export { DocxTransformer } from './docx.js';
 export { HtmlTransformer } from './html.js';
 export { MarkdownTransformer } from './markdown.js';
 export { NotionHtmlTransformer } from './notion-html.js';
+export { PdfTransformer } from './pdf.js';
 export { createAssetsArchive, download } from './utils.js';
 export { ZipTransformer } from './zip.js';

blocksuite/affine/widgets/linked-doc/src/transformers/pdf.ts

@@ -0,0 +1,32 @@
+import {
+  docLinkBaseURLMiddleware,
+  embedSyncedDocMiddleware,
+  PdfAdapter,
+  titleMiddleware,
+} from '@blocksuite/affine-shared/adapters';
+import type { Store } from '@blocksuite/store';
+
+import { download } from './utils.js';
+
+async function exportDoc(doc: Store) {
+  const provider = doc.provider;
+  const job = doc.getTransformer([
+    docLinkBaseURLMiddleware(doc.workspace.id),
+    titleMiddleware(doc.workspace.meta.docMetas),
+    embedSyncedDocMiddleware('content'),
+  ]);
+  const snapshot = job.docToSnapshot(doc);
+  if (!snapshot) {
+    return;
+  }
+  const adapter = new PdfAdapter(job, provider);
+  const { file } = await adapter.fromDocSnapshot({
+    snapshot,
+    assets: job.assetsManager,
+  });
+  download(file.blob, file.fileName);
+}
+
+export const PdfTransformer = {
+  exportDoc,
+};