fix(editor): add code block clipboard extension (#11731)

Close [BS-3109](https://linear.app/affine-design/issue/BS-3109/code-block-不支援-markdown-語法)

blocksuite/affine/blocks/code/src/index.ts

@@ -1,4 +1,5 @@
 export * from './adapters';
+export * from './clipboard';
 export * from './code-block';
 export * from './code-block-config';
 export * from './code-block-spec';