Support language and diff highlighting together

Support for both language and diff highlighting together is a popular feature request, as can be seen from the discussion https://github.com/github-linguist/linguist/discussions/5758 on the Linguist repo and [this Stack Overflow question](https://stackoverflow.com/questions/40883421/diff-syntax-highlighting-in-github-markdown).

This feature has already been implemented on other markdown-powered sites. The most straightforward syntax would be:

```diff
՝՝՝diff js
let fileIcon = document.querySelector("li.file-entry > span.icon");
+fileIcon.classList.remove("icon-file-text");
const disposable = addIconToElement(fileIcon, "/path/to/file.txt");
՝՝՝
```

Currently, this ignores the “js” part and only applies the “diff” syntax highlighting.