Add property-based tests (some are failing)

[seriyps]

Mostly based the generators on the typespecs in thoas module. Some of it borrowed from my old gist https://gist.github.com/seriyps/ffe0f5f5ba270fc4dd395208718e84d3

It fails on some input, CI would fail because of that:

• escape => html sometimes fails
• [{K, V}] representation of JSON objects seems to not be supported

Also I noticed typespec misses null

[lpil]

Thank you for this PR, and very cool to see how a bug has been found. I've left some minor notes inline and then we can merge or alternatively fix the bug then merge, depending on your preference.

[lpil]

Do these need to be exported? I could not see where they were used otherwise.

[seriyps]

Not really, but they were quite useful during development:

$ rebar3 as test shell
> proper_gen:pick(prop_thoas:json_term()).
{ok,#{<<>> => true,<<"I">> => 9.51697150738206,<<"TkxRF">> => 18,
      <<"Ylptz">> =>
          [false,
           <<230,167,155,200,141,238,134,152,56,7,230,191,141>>,
           false,-4.303609167549358,true,null,null],
      <<"p5kzkXK3pQ">> =>
          [<<238,158,129,54,204,185,195,129>>,
           null,null,false,false,-2.377673467827075,<<"yPCxZPihb">>,
           true]}}

[seriyps]

Thanks for the feedback. Waiting for your answer about typespec (and null as well) and formatter.

we can merge or alternatively fix the bug then merge, depending on your preference.

I don't have any strong preference. If you are fine with red CI on master, guess might make sense to merge this PR first since it is already here. But quite unlikely that I'm going to work on the bug fix unfortunately.

[lpil]

But quite unlikely that I'm going to work on the bug fix unfortunately.

Sounds like we want it merged then. The main branch doesn't deploy anywhere so a broken CI job is OK.