axisregistry: Axis order of user defined axes

I'm currently working on a tool which will update a designspace's instances based on our axis registry. Have we established an order yet for the user defined axes e.g CASL, GRAD etc?

For registered axes, we have the following order:
`opsz wdth wght ital` e.g `12pt Condensed Black Italic`

If we don't have an order, shall we just sort the user axes alphabetically?

For variable fonts we currently only include the wght and ital axes in the fvar instances so this request isn't relevant. However, for the static fonts, this info is important since a family which is constructed from multiple axes should be split into many different sibling families. If we take a look at Literata's [static fonts](https://github.com/google/fonts/tree/master/ofl/literata/static) we can see that it consists of four different families. Same situation for [Fraunces](https://github.com/google/fonts/tree/master/ofl/fraunces/static) as well.