refactor: remove duplicate mixin, move font mixin to typography
refactor: remove duplicate mixin, move font mixin to typography

CodeRichardJohnston/slate-demo,code-computerlove/quarry,code-computerlove/slate,cartridge/cartridge,code-computerlove/slate-cli,cartridge/cartridge-cli,CodeRichardJohnston/slate-demo,code-computerlove/slate