Also open/save JSON UFOs

[madig]

Fixes #859

EDIT(anthrotype):

This PR adds the ability to parse and dump UFOs serialized as JSON using ufoLib2. This is experimental and unofficial (not meant to be a replacement for UFO format!) and as such subject to change. Thus we do not recommended not to rely on this for long-term storage of font source data at the moment.
You can enable this by installing fontmake with the 'json' extra, e.g. pip install fontmake[json] (or fontmake[all] which installs all the other extras including this). This will in turn install two additional ufoLib2 dependencies, i.e. cattrs and orjson. Only cattrs is strictly required for this feature to work, but orjson makes (de)serialization much faster than pure-python built-in json module (on CPython at least).

Also note, the --save-ufo-as-zip option is deprecated (though it will continue to work) and replaced by a new --ufo-structure option that defaults to "package" (.ufo), and can take two additional values "zip" (for .ufoz) and "json" for ufoLib2's json-flavored UFOs.

[madig]

Hm. Maybe we want a (hard|optional) dep on ufoLib2[json,msgpack]?

[anthrotype]

Maybe we want a (hard|optional) dep on ufoLib2[json]?

hm maybe not for now. That will install not only cattrs but also orjson, which is a native dep.. They don't publish win32 wheels to PyPI for example, only win_amd64 (or fontmake CI windows runners currently test on win32 FWIW).
But even just installing cattrs (e.g. via ufoLib2[converters] extra) would be sufficient to enable this feature since, in absence of orjson, ufoLib2 will fall back to built-in json module.
Let's not advertise it too much maybe, and mark all this as experimental/unofficial/etc -- at least until we make a proper json schema for ufoLib2 classes, and we know what to make of this.

[anthrotype]

Maybe we want a (hard|optional) dep on ufoLib2[json]?

in the end, I added an optional 'json' to extras_require which fetches ufoLib2[json]