feat: synchronize file type icons with background

[ismailshak]

Since we're building a cache of icons for each file type, it prevents the icon from picking a new color when the background has been toggled between light and dark. I prefer/use color schemes that have dynamic highlighting based on background, and tend to toggle between them a lot depending on how bright it is around my monitor.

This PR makes the icon's color able to invalidate the cache

Comparing icons using default icon color

These are the correct colors for the OCaml icon as defined by the nvim-web-devicons spec, for each background;

This is how they render if I toggle the background without restarting neovim. I launched in light mode, then toggled to dark;