Markdown-It Mark Plugin

The `markdown-it-mark` package is a plugin for the `markdown-it` markdown parser, designed to extend its functionality by adding support for the HTML `<mark>` tag. It enables users to highlight text within their Markdown documents using the `==text==` syntax, which is then rendered as `<mark>text</mark>`. The current stable version is 4.0.0, last published approximately two years ago, indicating a mature and stable project likely in maintenance mode rather than rapid feature development. This plugin is highly focused, providing a straightforward implementation for text highlighting that adheres to similar rules as CommonMark emphasis. Its key differentiator is its seamless integration with the `markdown-it` ecosystem, offering a lightweight solution for a common formatting need without introducing complex parsing behaviors.