Fix lost file comment when it precedes declare(strict_types=1)

[xHeaven]

Extractor::extractAll() fails to extract the file-level doc comment when it appears before declare(strict_types=1). This means PhpFile::fromCode() silently drops the comment during a print-then-reparse round-trip.

We discovered it in tempest/framework where our generation package round-trips through fromCode().

The PsrPrinter (and Printer) places the file comment before the declare statement:

<?php

/**
 * This file has been generated.
 */

declare(strict_types=1);

namespace App;

But extractAll() (changed here) skips the Declare_ node and only checks the next statement for a doc comment. Since the PHP parser attaches the comment to the Declare_ node it precedes, the comment is never found.

The added tests in the above commit also only cover cases where the comment comes after a singleline declare, not before it.

Quick reproduction:

$file = new PhpFile();
$file->setStrictTypes();
$file->setComment('Generated file.');
$file->addNamespace('App')->addClass('Foo');

$code = (new PsrPrinter())->printFile($file);
$roundTripped = PhpFile::fromCode($code);

$roundTripped->getComment(); // null, expected 'Generated file.'

I've added a regression test as well to cover this case.