Support language and diff highlighting together

[hb20007]

Support for both language and diff highlighting together is a popular feature request, as can be seen from the discussion github-linguist/linguist#5758 on the Linguist repo and this Stack Overflow question.

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

՝՝՝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.