Better support for three party highlight plugins

[xihale]

Check List

• [X] I have already read Docs page.
• [X] I have already searched existing issues.

Feature Request

At https://github.com/hexojs/hexo/blob/master/lib/plugins/tag/include_code.ts#L76

return `<pre><code>${code}</code></pre>`;

It's better using this case:

return `<pre><code class="language-${lang}>${code}</code></pre>`;

Others

I am a user use neither prismjs nor hljs, and meanwhile I am a developer for hexo-shiki which is a simple glue code connecting shikijs and hexo. But when I use include_code tag, I got some error, with some debug, I think modify this line is the better solution(Just transfer the language parameters), for me and for any others who want to develop three party highlight plugin!

[xihale]

The problem is for I still use old hexo version.