[RFC] Add a way to opt-in ext/dom spec compliance (#13031)

RFC: https://wiki.php.net/rfc/opt_in_dom_spec_compliance

Fix GH-8996: DOMNode serialization on PHP ^8.1

PHP 8.1 introduced a seemingly unintentional BC break in ca94d55a19 by
blocking the (un)serialization of DOM objects.
This was done bec

Fix GH-8996: DOMNode serialization on PHP ^8.1

PHP 8.1 introduced a seemingly unintentional BC break in ca94d55a19 by
blocking the (un)serialization of DOM objects.
This was done because the serialization never really worked and just
resulted in an empty object, which upon unserialization just resulted in
an object that you can't use.

Users can however implement their own serialization methods, but the
commit made that impossible as the ACC flag gets passed down to the
child class. An approach was tried in #10307 with a new ACC flag to
selectively allow serialization with subclasses if they implement the
right methods. However, that was found to be too ad hoc.

Instead, let's abuse how the __sleep and __wakeup methods work to throw
the exception instead. If the child class implements the __serialize /
__unserialize method, then the throwing methods won't be called.
Similarly, if the child class implements __sleep and __wakeup, then
they're overridden and it doesn't matter that they throw.

For the user, this PR has the exact same behaviour for (sub)classes that
don't implement the serialization methods: an exception will be thrown.
For code that previously implemented subclasses with these methods, this
approach will make that code work again. This approach should be both BC
preserving and unbreak user's code.

Closes GH-12388.

For the test:
Co-authored-by: wazelin <contact@sergeimikhailov.com>

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Mark DOM classes as not serializable