Support reading and writing JSON-ified UFOs

[madig]

Like ufoLib2. Would make it easier to process UFOs in build systems.

[rsheeter]

Is UFO-in-json specified somewhere? Can the designspace be json?

[madig]

It's specified in ufoLib2 code behavior :) The serde code is in e.g. https://github.com/fonttools/ufoLib2/blob/main/src/ufoLib2/serde/json.py and since the dataclasses in ufoLib2 mirror the UFO structure, you can just serialize a ufoLib2.Font to JSON.