Added examples of typical errors in JSON

[Alexandre887]

JSON methods, toJSON

let json = `{
  *!*name*/!*: "John",                      // mistake: property name without quotes
  "surname": *!*'Smith'*/!*,                // mistake: single quotes in value (must be double)
  *!*'isAdmin'*/!*: false,                  // mistake: single quotes in key (must be double)
  "birthday": *!*new Date(2000, 2, 3)*/!*,  // mistake: no "new" is allowed, only bare values
@@"gender": "male"*!* */!*                  // mistake: missing comma after non-last property @@
@@"friends": [0,1,2,3]*!*,*/!*              // mistake: there should not be a comma after the last property @@
}`;

In practice, many novice developers, following the rule that in JavaScript there can be a "trailing comma" in an object, try to reproduce the same in JSON. But a trailing comma in JSON makes it invalid. This is a very widespread stuff, so I think we should add such an example to "typical mistakes".

Also, many people forget that a comma should be after each property except the last one, which also causes JSON to become invalid. I also added such an example.