fix(markdown/plugins/containers): use Token.attrJoin for rendering additional className

[TheNorthMemory]

Description

while there is an additional css className, for eg: .my-class onto the custom containers, vitepress is crashed with sth like:

11:19:56 PM [vitepress] Internal server error: Duplicate attribute.

The markdown content like below,

::: info {.my-class id=test}
hello
:::

Using the Token.attrJoin method is solved this problem, rendering as below:

<div class="my-class info custom-block" id="test">
  <p class="custom-block-title">INFO</p>
  <p>hello</p>
</div>

Linked Issues

Additional Context

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Tip

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[TheNorthMemory]

Hi there, a live usage scenario is here https://wechatpay.im/openapi/v3/merchant-service/complaints , the markdown content as below,

::: danger :no_entry_sign: {.im-deprecated}

本接口服务已于 `2020.11.27` (北京时间)下线，文档仅做留存参考。

:::

Hopes it shall be merged. @brc-dd