Add defmt support

[kaidokert]

For convenience - JsonNumber + String should just have defmt outputs

[dario23]

i stumbled on this while looking for defmt support for printing the picojson::Error type; would you be open to a merge request adding this?

[kaidokert]

Absolutely, if it's feature-gated i'd be very happy

[kaidokert]

Released in 0.2.3