no utf surrogate in json.dump

[josojo]

I noticed during testing, that uft surrogates are being used.

E.g. in the mathlib notations like this one:

@[inherit_doc]

# scoped notation "𝓟" => Filter.principal

would only yield " \ud835\udcdf" instead of "𝓟" and this would not be consumed correctly by the REPL env.

[yangky11]

Merging into dev and running unit tests. Thanks!