Default to rich diff each SVG file in a pull request #1932

I've used a similar approach as the one in #1924. With MutationObserver we can watch changes in the DOM for files that are loaded asynchronously and added to the page a posteriori.

This functionality can be extended to other files (markdown ones, for example): just add the intended extension to the array defined in default-to-rich-diff.tsx::7.

Example of a PR with SVG files: https://github.com/elementary/icons/pull/792 (as the issue's OP said "pretty much any PR at icon repos, such as https://github.com/elementary/icons/pulls")

Closes #1599