Add support for building GDEF LigatureCarets from "caret_X" anchors

[anthrotype]

Apparently Glyphs.app builds these now, and @schriftgestalt said they are implemented on Apple platform.

see discussion in unified-font-object/ufo-spec#89

[anthrotype]

I had forgotten that we do support these in fontmake, though only when building from glyphs sources:

https://github.com/googlefonts/glyphsLib/blob/9c12dc70c8d13f08d92b824e6710f6e3bb5037bb/Lib/glyphsLib/builder/features.py#L183

Still, would be nice to have a feature writer that would output these in features.fea from UFO sources

[benkiel]

This is now in the UFO spec: unified-font-object/ufo-spec#90

[madig]

If this gets implemented, it would be nice if it could be appended to GDEF tables defined in features.fea. A project I'm working on defines an explicit GDEF GlyphClassDef but I'd love to leave out explicit LigatureCaretByPos statements because those are master/instance specific.

[moyogo]

Fixed in #480.