EZP-23646: implement converter with tests

This PR resolves issue https://jira.ez.no/browse/EZP-23646

As local namespaces as used by legacy EZXML for temporary paragraphs are inherited by descendants, they are not preserved in XSL transformation even if explicitly added.
This needs to be handled by conversion pass that will use PHP's DOM API to add local namespace as an attribute, on paragraphs identified by temporary attribute.

#### TODOs
- [ ] adapt core conversion for needed attribute markers