Adds serialization API

[redvers]

Add document serialization APIs to high-level Pony API.

This adds serialize() and saveToFile() methods to Xml2Doc, enabling
XML documents to be saved to strings or files with optional formatting
and encoding control.

Implementation:
- Added serialize() method using xmlDocDumpFormatMemoryEnc for in-memory
  serialization with format and encoding options
- Added saveToFile() method using xmlSaveFormatFileEnc for direct file
  output
- Added xmlFree wrapper to raw API for proper memory management
- Both methods support pretty-printing and multiple character encodings
  (UTF-8, ISO-8859-1, UTF-16, etc.)

Testing:
- Added 6 comprehensive test cases covering round-trip serialization,
  formatting options, file I/O, encoding support, modified document
  serialization, and error handling
- All 33 tests pass successfully

This feature enables both modifying existing XML documents and provides
the foundation for creating new XML documents from scratch.

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

[redvers]

Hi @redvers,

The changelog - added label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 25.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.