Add image zoom feature

[alshedivat]

Image zooming (https://github.com/kingdido999/zooming/) could be particularly useful for project pages and blog posts.

[Jongwoo-Shim]

@alshedivat I tried playing around with the zoom library that you provided, and I think that we'd be better off using a different one. When trying to integrate it with the existing dark-mode I encountered multiple bugs.

• Issues with opacity when trying to use css-variables.
• Not running until the mode toggle is first hit. This is despite the listener being initialized during the theming initialization.
• Say hello to stretchy al :
  
  • This happens when repeatedly zooming in/out the image.

I've been looking into medium-zoom instead. If that doesn't work either, then I think maybe just raw js might be the best option.