…classe to ease its usage (lyrixx)

This PR was merged into the 3.2-dev branch.

Discussion
----------

[ExpressionLanguage] Move the ::dump method to the Node classe to ease its usage

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Example (for @nicolas-grekas  )

```php
<?php

require __DIR__.'/vendor/autoload.php';

use Symfony\Component\ExpressionLanguage\ExpressionLanguage;
use Symfony\Component\ExpressionLanguage\Node\NameNode;

$el = new ExpressionLanguage();

$expression = $el->parse('a + b + 12', ['a','b']);

function dump_with_highlight($node)
{
    $dump = '';

    foreach ($node->toArray() as $v) {
        if (is_scalar($v)) {
            $dump .= $v;
        } elseif ($v instanceof NameNode) {
            $dump .= sprintf('`%s`', dump_with_highlight($v));
        } else {
            $dump .= dump_with_highlight($v);
        }
    }

    return $dump;
}

echo dump_with_highlight($expression->getNodes());
```
Result:

```
((<strong>a</strong> + <strong>b</strong>) + 12)
```

Commits
-------

b6d0050 [ExpressionLanguage] Move the ::dump method to the Node classe to ease its usage