Check existence of childTypes method

[hctorres02]

When using enums as array keys in the properties, PHP throws a syntax error, as the interpretation is considered to be dynamic. To work around this, if the values are inside a method, they will be interpreted as part of the invocation.

The method will take precedence over the property.

Before:

protected $childTypes = [
  FooEnum::BAR->value => Bar::class,
  FooEnum::BAZ->value => Baz::class,
];

After:

protected function childTypes(): array
{
  return [
    FooEnum::BAR->value => Bar::class,
    FooEnum::BAZ->value => Baz::class,
  ];
}

[driftingly]

It looks like this issue is fixed in PHP 8.2 https://wiki.php.net/rfc/fetch_property_in_const_expressions

I'll merge in for supporting earlier PHP versions as well as provide flexibility beyond Enums.

[driftingly]

Thanks @hctorres02 🙌