json module format functions for key-value lists

[isvilen]

Existing json:encode_key_value_list/2 and json:encode_key_value_list_checked/2 can be used to encode key-value lists while preserve their ordering. It will be great if json module exports corresponding -spec format_key_value_list([{term(), term()}], Encode::formatter(), State::map()) -> iodata(). and -spec format_key_value_list_checked([{term(), term()}], Encode::formatter(), State::map()) -> iodata(). for generating formatted JSON from key-value lists.

[dgud]

A PR would increase the possibility of that to happen.