Add retry helper stub

[Daanra]

Adds a stub for the retry helper with more accurate PHPDoc annotations.

This ensures that the following things will be seen as invalid by Larastan:

public function do(): int
{
   return retry(2, function (): string { // int and string mismatch
      return 'hello';
   });
}

retry(2, function ($param): int { // Retry does not accept a parameter in its closure
      return 5;
});

[canvural]

The actual filename is helpers.stub but it should be Helpers.stub. Other than that looks good 👍

[canvural]

Thank you!

[szepeviktor]

maybe a file with procedural code should not have uppercases