Fix FP S3800 (function-return-type): Make the exception of returning "this" work

[yassin-kammoun-sonarsource]

The rule reports functions with multiple return statements of different types. Also, it is supposed to make exceptions to functions whose one of its return statements returns this. However, the exception is not working, and a false positive is raised on the following snippet:

String.prototype.foo = function()
{
    if (this.length == 10)
        return this.substring(5);
    
    return this;
}

Reported in https://community.sonarsource.com/t/javascript-s3800-fp-when-working-on-prototype-function/102118