support exporting vertical kerning to UFO

There's a `kerningVertical` dict in .glyphs sources, which glyphsLib can parse and serialize from/to .glyphs files. We need to find a way to export that to the UFO in a way that fontmake can then use with ufo2ft (https://github.com/googlefonts/ufo2ft/issues/668) to compile vkrn feature.

Start by tracing the kerningVertical parsing and export path from .glyphs sources into UFO data. Compare the exported representation with what fontmake and ufo2ft need for vkrn compilation, using ufo2ft issue #668 as context. Done means vertical kerning survives export in a form the downstream toolchain can consume.

Written by the indexing model from the issue text.