Write designspace v5 STAT labels

It does not seem that glyphsLib writes any kind of STAT table information to extracted designspaces (see https://github.com/googlefonts/ufo2ft/issues/723). With support for designspace v5 `` being available in fonttools ([docs](https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#labels-element-top-level)), glyphsLib should probably write, at the very least, labels for axes and defined instances.

Start by tracing glyphsLib's extracted designspace output and compare its current STAT-related output with the fontTools designspace v5 labels documentation and ufo2ft issue #723. Done means extracted designspaces include labels for axes and defined instances, with coverage for the supported cases verified by the project's tests.

Written by the indexing model from the issue text.