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/blocks/list/src/utils/get-list-icon.ts

@@ -1,4 +1,5 @@
 import type { ListBlockModel } from '@blocksuite/affine-model';
+import { getNumberPrefix } from '@blocksuite/affine-shared/utils';
 import {
   BulletedList01Icon,
   BulletedList02Icon,
@@ -11,8 +12,6 @@ import {
 } from '@blocksuite/icons/lit';
 import { html } from 'lit';
 
-import { getNumberPrefix } from './get-number-prefix.js';
-
 const getListDeep = (model: ListBlockModel): number => {
   let deep = 0;
   let parent = model.store.getParent(model);

blocksuite/affine/blocks/list/src/utils/get-number-prefix.ts

@@ -1,52 +0,0 @@
-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
-export 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 i in lookup) {
-    while (num >= lookup[i]) {
-      romanStr += i;
-      num -= lookup[i];
-    }
-  }
-  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}.`;
-}