Disallow the use of $_Request

[Morerice]

Create a new sniff which will listen on the variable constant ($). If the $_REQUEST super global is detected, add the following error:

The $_REQUEST super global should not be used. Use $_GET, $_POST or $_COOKIE instead

This implements #2373

[gsherwood]

Thanks a lot for this contribution.

I decided to rename the sniff to Generic.PHP.DisallowRequestSuperglobal and change references to "super global" to "superglobal" as per the PHP manual. Other than that - I left it alone. Thanks for the docs as well.

[Morerice]

@gsherwood You're welcome and thanks for guiding me on the first pr on this project. I hope I have time to tackle some other issues in the future.