Add promote_namespaces function

[veewee]

Summary

• Adds promote_namespaces as both a Manipulator and Configurator
• Moves prefixed namespace declarations (xmlns:foo="...") from child elements to the document root, preserving original prefix names
• Handles conflicts: if the same prefix maps to different URIs, the first in document order is promoted; subsequent conflicting declarations are left in place
• Default namespace declarations (xmlns="...") are not promoted

Unlike optimize_namespaces, this does not rename prefixes.

Context

Some SOAP servers (e.g. Microsoft Business Central) run XSD validation that requires namespace declarations on the envelope element, not scattered on child elements. The XML is semantically identical, but their validation rejects inline declarations.

Ref: php-soap/encoding#48

Test plan

• 13 manipulator test cases covering: no-op, already on root, child-to-root, duplicates, mixed namespaces, root-vs-child conflict, child-vs-child conflict, deep nesting, default namespace, empty namespace, namespaced attributes, and a realistic SOAP envelope
• 4 configurator test cases verifying the wrapper works during Document::fromXmlString()
• Full test suite passes (688 tests)
• Psalm passes with no errors