Feature: Support "diff"

[mifi]

Like this:

-oldCode()
+newCode()

```diff

[sindresorhus]

I have updated the fixture to include diff codeblock, but GitHub has changed their CSS and it's not in sync with the GitHub Markdown API, so syntax highlighting stopped working and blocks are missing background, so I cannot ship it as is.

https://github.com/sindresorhus/generate-github-markdown-css/commits/main