Create a utility for highlight.js line numbers

As part of the unified highlighting architecture (#7788), we need a simple, environment-agnostic utility to add line number markup to HTML generated by highlight.js. This utility must work in both the browser and Node.js environments.

The current browser-based plugin is not compatible with Node.js as it relies on the DOM.

Plan

1. Create a new utility , following the pattern of .
2. The file will be located at .
3. It will be a plain JavaScript object (not a class) wrapped with .
4. It will provide a method that takes an HTML string (from ) and returns a new HTML string with a table structure for line numbers.
5. The implementation will be based on the Node.js-compatible version discussed in this GitHub issue, but will have no external dependencies (like lodash).
6. The generated HTML structure should be compatible with the existing SCSS for line numbers.

SCSS

// highlight-js line numbers styles
.hljs-ln {
    border-collapse: collapse;
    td {
        padding: 0;
    }

    /* for block of numbers */
    td.hljs-ln-numbers {
        -webkit-touch-callout: none;
        -webkit-user-select: none;
        -khtml-user-select: none;
        -moz-user-select: none;
        -ms-user-select: none;
        user-select: none;

        text-align: center;
        border-right: 1px solid #717171;
        vertical-align: top;
        padding-right: 5px;
    }

    /* for block of code */
    td.hljs-ln-code {
        padding-left: 10px;
    }
}

.hljs-ln-n:before {
    content: attr(data-line-number)
}