nixos: unify virtual console options

This commit moves all the virtual console related options
to a dedicated config/console.nix NixOS module.

Currently most of these are defined in config/i18n.nix
with a "console" prefix like `i18n.consoleFont`,
`i18n.consoleColors` or under `boot` and are implemented
in tasks/kbd.nix.
Since they have little to do with actual internationalisation
and are (informally) in an attrset already, it makes sense to
move them to a specific module.