DisallowDefineSniff should only check for top-level define

`DisallowDefineSniff.php` adds and error (!) everytime a `define` is encountered and the reason (which I fully agree to) is that `const` should be preferred.

However, in PHP `const` is not allowed inside condition blocks, because those are only checked at runtime. For example a snippet like the following is invalid and throws a parse error:

```php
if (!defined('FOO')) {
const FOO = true;
}
```

Another limitation of `const` is that it does not accept expression as constant name. For example following is also invalid and throws a parse error:

```php
foreach(['FOO', 'BAR'] as $name) {
const $name = true;
}
```

So there are cases in which the usage of `define` is really necessary, and pretty much it turns out to be anytime the _level_ of `const` token is not `1`.

If you agree, I could send a PR for this.

Start by reading DisallowDefineSniff.php and trace how it determines the nesting level of each define call. Check the conditional and dynamic-name examples from the issue, then confirm that only top-level define usage is reported while nested usage remains allowed.

Written by the indexing model from the issue text.