[Feature Request] Support code element's language attribute when parsing XML comment

[filzrev]

Is your feature request related to a problem? Please describe.

As reported by #9145.
When <code> tag is used inside XML doc comment.
lang-csharp style is automatically applied.

It seems older version of docfx don't set this style and highlight.js's auto language detection feature is used.

Describe the solution you'd like
Sandcastle support language attribute on <code> element.
If language attribute is set. It can use this value to select language for syntax highlighting.

Some language (xaml) is not natively supported by hightlight.js
So it need mapping to similar language(xml).

Describe alternatives you've considered
Don't explicitly set lang-* styles by default.
Instead use highlight.js's auto language detection.

[filzrev]

By PR #9159. It can specify language for syntax highlighting.
e.g. <code language="{languageId}">

Specified languageId must be included by is hightlight.js supported language
Otherwise. syntax highlighting is not executed.

For XAML file. It need to customize template's main.js file to register language alias.

  configureHljs: function (hljs) {
    hljs.registerAliases('xaml', { languageName: 'xml' });
  }