feat: add an expressive-code plugin to support marks in code block

[Sudrut]

With the plugin one can specify part of code block as "is a syntax notion", "is exposition only", or "is optional" rendered in cppref style.

Enable this feature by adding cxx-mark after the language name of code block's opening fence. When enabled, inside the code block type /*$s:content*/ for syntax notation, /*$expos:content*/ for exposition-only things and /*$opt*/ for an optional mark. The syntax /*$...*/ above is chosen just for convenience.

For example, provided the following DeclDoc component:

<DeclDoc>
  <Decl slot="decl">
    ```cpp cxx-mark
    int main(/*$expos: implementation-defined, see below, or exposition only */) { /*$s:body*/ }
    /* normal C-style comment *//*$opt*/
    ```
  </Decl>

  A main function of implement-defined type, returning `int`.
</DeclDoc>

Output:

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Project	Deployment	Preview	Comments	Updated (UTC)
cppdoc	Error			Dec 6, 2025 7:48am

[Lancern]

Could you also update the development documentation at /development/guide/doc-everything and add usage instructions for your plugin?

[Sudrut]

Could you also update the development documentation at /development/guide/doc-everything and add usage instructions for your plugin?

Ok I will try to add instructions to that page in a section named "Code block inline markers". I couldn't figure out a better name for it.

[Sudrut]

The documentation is added. Sorry I'm terribly sick and will be unable to work for a period of time.

[Lancern]

LGTM!