Sass mixins include multiple axes files

[jwr1]

When using the new sass mixins with variable fonts, the inclusion of multiple axes is not handled correctly. Instead of using the file that best matches the selected axes (I.e. if wght, ital and another are selected, standard should be used), multiple wght+axis files are included. Also, using all as the axes argument currently always resolves to full, but not all fonts have full, they might only have standard.