Syntax Highlighting for "JSON with Comments" a.k.a. jsonc

Problem

In JSON there are no comments allowed. This is also honored by Github syntax
highlighting in fenced code blocks:

{
    // Note: there might be additional sizes available
    "sizes": [ "S", "M", "L", "XL", "XXL" ]
}

There is also "JSON with Comments" or jsonc (see Github language list)
which allows C-style comments

{
    // Note: there might be additional sizes available
    "sizes": [ "S", "M", "L", "XL", "XXL" ]
}

This is prominently used in tsconfig.json, jsconfig.json and
vscode configuration files.

It would be nice to have syntax highlighting support for such code blocks.

A code fence with jsonc should be highlighted:

```jsonc
{
    // Note: there might be additional sizes available
    "sizes": [ "S", "M", "L", "XL", "XXL" ]
}
```

Proposed Solution

• Support for "JSON with Comments" in highlight.js: highlightjs/highlight.js/issues/2016
  • this adds rendering of C-style comments to normal JSON
• jsonc alias also available: highlightjs/highlight.js/issues/4018
  • seemingly not yet contained in a stable release

Notes

If #2313 is considered, Prism seems to also render comments in "normal" JSON, but there is no jsonc alias.

The issue does not name an mdBook file or test. Start by locating mdBook's syntax-highlighting integration and reviewing the linked highlight.js issues, then verify that a fenced jsonc block renders comments and JSON syntax as highlighted code.