🐛 Gracefully handle invalid dates

[homersimpsons]

(found in symfony-tools/docs-builder#179)

When parsing the following yaml:

date: 6418-75-51

symfony/yaml will throw an exception:

$ php main.php
PHP Fatal error:  Uncaught Exception: Failed to parse time string (6418-75-51) at position 6 (5): Unexpected character in /tmp/symfony-yaml/vendor/symfony/yaml/Inline.php:714
Stack trace:
#0 /tmp/symfony-yaml/vendor/symfony/yaml/Inline.php(714): DateTimeImmutable->__construct()
#1 /tmp/symfony-yaml/vendor/symfony/yaml/Inline.php(312): Symfony\Component\Yaml\Inline::evaluateScalar()
#2 /tmp/symfony-yaml/vendor/symfony/yaml/Inline.php(80): Symfony\Component\Yaml\Inline::parseScalar()
#3 /tmp/symfony-yaml/vendor/symfony/yaml/Parser.php(790): Symfony\Component\Yaml\Inline::parse()
#4 /tmp/symfony-yaml/vendor/symfony/yaml/Parser.php(341): Symfony\Component\Yaml\Parser->parseValue()
#5 /tmp/symfony-yaml/vendor/symfony/yaml/Parser.php(86): Symfony\Component\Yaml\Parser->doParse()
#6 /tmp/symfony-yaml/vendor/symfony/yaml/Yaml.php(77): Symfony\Component\Yaml\Parser->parse()
#7 /tmp/symfony-yaml/main.php(8): Symfony\Component\Yaml\Yaml::parse()
#8 {main}
  thrown in /tmp/symfony-yaml/vendor/symfony/yaml/Inline.php on line 714

This is because the "month" is invalid. Fixing the "month" will trigger about the same issue because the "day" would be invalid.

With the current change it will gracefully handle the value as a string:

$ php main.php
/tmp/symfony-yaml/main.php:8:
array(1) {
  'date' =>
  string(10) "6418-75-51"
}

[homersimpsons]

I should create this Pull Request in symfony/symfony.