Add support for PhpParser v5 for PHP 7.4+

[IanDelMar]

This PR adds support for nikic/php-parser v5 for PHP 7.4+, while retaining compatibility with v4 for PHP 7.3.

Key Changes

• The parts property of PhpParser\Node\Name was deprecated in v4 and removed in v5. It has been replaced with a call to getFirst(), which is available in both major versions.
• The create() method and the PREFER_PHP7 constant were removed from PhpParser\ParserFactory in v5. This PR uses the createForNewestSupportedVersion() method instead, which is available in both v4 and v5. According to the upgrade guide, this method is "roughly equivalent to PREFER_PHP7 behavior".

Testing

The changes were tested with php-stubs/wordpress. Output under v5 (5.4.0) differs slightly but improves correctness:

• No longer adds incorrect slashes.
• Preserves the correct order of final and visibility modifiers.

Closes #25
Closes #31

[szepeviktor]

Thank you.