Update flake inputs for newer version of Iosevka
Fix lower-chi in build plan: semi-chancery-straight ->
semi-chancery-straight-serifless
Rename default.nix -> derivation.nix
Add Iosevka Etoile back again, simply slab variant of Aile
Move names to separate files so they can be reused in both font
definition and names output of flake
