Improve ConfigLoader::PHP_CONST_REGEX

[Wirone]

Continuation from #4441 🙂

Differences between proposed regex and actual change in the PR:

• added support for numbers in constants
• instead of \\ in first group I had to do \\\\ because somehow tests were failing: class' FQCN was not properly caught and was causing syntax issues, e.g.:

  Symfony\Component\Yaml\Exception\ParseException: Unexpected characters near "\ConfigTransformer\Tests\Converter\ConfigFormatConverter\YamlToPhp\YamlToPhpTest::TEST" at line 2 (near "class_constant: "%const(Symplify)%"\ConfigTransformer\Tests\Converter\ConfigFormatConverter\YamlToPhp\YamlToPhpTest::TEST").

Regex URL was updated, but currently it does not contain \\\\ change (works without it).

[TomasVotruba]

Thank you 👏 👍