Fix JsonParser not able to parse "\u0000"

[Genteure]

Currently JSON.parse does not handle \u0000 correctly:

var engine = new Engine();
engine.SetValue("log", new Action<object>(Console.WriteLine));
engine.Execute(@"js code below");

const null_text = '{"a":"\\u0000"}';
const null_obj = JSON.parse(null_text);
log(null_text);
log(null_obj.a.length);
log(null_obj.a);

const a_text = '{"a":"\\u0061"}';
const a_obj = JSON.parse(a_text);
log(a_text);
log(a_obj.a.length);
log(a_obj.a);

Expected output:

{"a":"\u0000"}
1

{"a":"\u0061"}
1
a

Current output:

{"a":"\u0000"}
5
u0000
{"a":"\u0061"}
1
a

This PR removed unneeded check for escaped characters, and added tests to verify escaped characters are correctly parsed.

[lahma]

Looks good, thank you!