[9.x] Adds Str::isJson

[ecrmnn]

Adds isJson() to Str and Stringable.

Str::isJson($data); 
//=> boolean

Str::of($data)->isJson()
//=> boolean

str($data)->isJson();
//=> boolean

Uses JSON_THROW_ON_ERROR flag introduced in PHP 7.3 under the hood.

[ecrmnn]

@florisbosch

By it self '' isn't valid JSON.

json_decode('', false, 512, JSON_THROW_ON_ERROR);

// [THROWS] JsonException with message 'Syntax error'

null isn't valid because this is a string helper, therefore null will never be valid because it's not a string