Support restricted variable axis range sub-spaces at Level 4

[chrissimpkins]

Sub-spacing a design axis to a range that does not include the default value requires compile-time editing of the default axis location. This is not currently supported in the fonttools library used to compile fonts in this project.

We intend to support this functionality when it is available in the library.

Related issue on the fonttools tracker: https://github.com/fonttools/fonttools/issues/2157