Add anki support

[dastarruer]

With this recent pr: NixOS/nixpkgs#404045 anki can now be installed with addons. These addons can be configured using <add-on>.withConfig.
To theme Anki, you need to use the Recolor addon. An example of someone using stylix to achieve this can be found here: https://discourse.nixos.org/t/how-to-configure-addons-installed-with-anki-withaddons/66375/10
However, as they state their theme is a bit janky. If stylix could support an official theme, that would definitely be great.

[trueNAHO]

This is already WIP: #1801