💄 Dark and Light Image Variants

[hayesall]

This picks up pull request #1991 but implements a different light/dark mode variant that should be easier in markdown.

Writers can append #only-light or #only-dark to the end of an image path:

![rust logo standard edition](images/rust-logo-blk.svg#only-light)
![rust logo dark mode](images/rust-logo-blk-dark.svg#only-dark)

... which renders:

short-dark-light-mode-variant-demo.mp4

[ehuss]

Thanks, this looks interesting!

I'm a little concerned about hard-coding the themes, as this won't work if the user adds their own themes. The general.css file should not have any theme values in it. Is it at all possible to make it so that the CSS rules can be based on the color-scheme of the theme? My quick searching didn't turn up anything useful, but perhaps the color-scheme could be exposed as a data attribute on the root tag? That might be a little more complex than this solution since the javascript code would need to update it when it updates the class list. I'm just wondering what kinds of options there are.