Consider partial instantiation in addition to full, static instantiation

[chrissimpkins]

"Partial instantiation" = final file includes variable design axis support for at least one axis, other axes are instanced (not currently supported in Slice)
"Static instantiation" = final file has no variable axes, all axes are instanced (current approach in Slice)

[chrissimpkins]

Supported for TTF in varLib.instancer.

https://fonttools.readthedocs.io/en/latest/varLib/instancer.html

This currently supports partial instantiation that creates a static design axis value while leaving one or more variable design axes as is with the full design axis range. There are plans on the fontTools side to support range limitations through a MIN:MAX range syntax, but this is not currently supported in the library.

[chrissimpkins]

Support added in 83bca6e

Will be released in v0.5.0

[chrissimpkins]

I revised the approach to something that is much more straightforward. We will use the following definitions for variable and instanced axes:

• if the axis entry field is empty, it remains a variable axis
• if the axis entry field is defined, the axis is sliced at that instance value

I like this approach. It is simple and seems intuitive to me.

This reverts the previous use of "var" or "variable" entry strings for anyone who happened to work with source in the main branch.

I will release this support in v0.5.0.

[chrissimpkins]

Added in v0.5.0