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/src/utils/index.ts

@@ -15,6 +15,7 @@ export * from './insert';
 export * from './is-abort-error';
 export * from './math';
 export * from './model';
+export * from './number-prefix';
 export * from './popper-position';
 export * from './print-to-pdf';
 export * from './reference';

blocksuite/affine/shared/src/utils/number-prefix.ts

@@ -0,0 +1,53 @@
+function number2letter(n: number) {
+  const ordA = 'a'.charCodeAt(0);
+  const ordZ = 'z'.charCodeAt(0);
+  const len = ordZ - ordA + 1;
+  let s = '';
+  while (n >= 0) {
+    s = String.fromCharCode((n % len) + ordA) + s;
+    n = Math.floor(n / len) - 1;
+  }
+  return s;
+}
+
+// Derive from https://gist.github.com/imilu/00f32c61e50b7ca296f91e9d96d8e976
+function number2roman(num: number) {
+  const lookup: Record<string, number> = {
+    M: 1000,
+    CM: 900,
+    D: 500,
+    CD: 400,
+    C: 100,
+    XC: 90,
+    L: 50,
+    XL: 40,
+    X: 10,
+    IX: 9,
+    V: 5,
+    IV: 4,
+    I: 1,
+  };
+  let romanStr = '';
+  for (const [key, value] of Object.entries(lookup)) {
+    while (num >= value) {
+      romanStr += key;
+      num -= value;
+    }
+  }
+
+  return romanStr;
+}
+
+function getPrefix(depth: number, index: number) {
+  const map = [
+    () => index,
+    () => number2letter(index - 1),
+    () => number2roman(index),
+  ];
+  return map[depth % map.length]();
+}
+
+export function getNumberPrefix(index: number, depth: number) {
+  const prefix = getPrefix(depth, index);
+  return `${prefix}.`;
+}