feat(gen): accept a JSON from the LVGL JSON generator

[liamHowatt]

For lvgl/lvgl#6068

This is a backwards-compatible change that allows gen_mpy.py to accept a JSON from the LVGL JSON generator.

Currently it only uses the JSON to find the user_data member of private structs.

[liamHowatt]

Blocking lvgl/lv_micropython#84

[kdschlosser]

for a minute there you had me thinking that the gen_mpy script had been rewritten to use the JSON output instead of Pycparser directly.

[kisvegabor]

I would be ok with making the MP binding fully work based on the JSON. But let's proceed step by step. :)