feat(editor): extract linked doc widget package (#11589)

Close [BS-2738](https://github.com/toeverything/AFFiNE/pull/11589)

blocksuite/playground/apps/starter/data/synced.ts

@@ -1,5 +1,5 @@
-import { MarkdownTransformer } from '@blocksuite/affine/blocks/root';
 import { Text, type Workspace } from '@blocksuite/affine/store';
+import { MarkdownTransformer } from '@blocksuite/affine/widgets/linked-doc';
 
 import type { InitFn } from './utils';