* Make sure core module has number 0
Some places, possibly also outside PHP, assume the core extension has
module number 0. After 8a812c3fda this wasn't the case anymore as
reported in [1]. Fix it by changing how the next module ID is computed.
[1] https://github.com/php/php-src/pull/12246#issuecomment-1731432377
* Add assertion
* Add test
