feat: add plus-darker support for mix-blend-mode

[clementinelove]

This PR adds 'plus-darker' support to mix-blend-mode.

One PR already added 'mix-blend-plus-lighter' (#8288) , but there should also be 'mix-blend-plus-darker' since blend mode also has a 'plus-darker' value that could be added last time.

mix-blend-mode on MDN lists all possible values

From what I know, it seems only Safari supports plus-darker at the moment since this style is used heavily in macOS's vibrant visual style.