Add experimental PHPantom lsp support for PHP

[cappuc]

Summary

PHPantom is a new language server for PHP written in rust that provides better performance and lower resource utilization that other solutions.

Note

This implementation is done with AI assistance since I'm not a python export. I hope the code is good enough.
I've been testing serena with php_phpantom for a few days on real projects and it seems to work well.

Changes

• Add php_phpantom as an experimental PHP language backend powered by PHPantom-dev/phpantom_lsp.
• Wire the new backend into language selection, runtime dependency resolution, and PHP test coverage.
• Document the new backend in the changelog and PHP language docs.

Testing

• Added targeted PHPantom integration coverage for diagnostics, symbol lookup, rename behavior, hover fallback, and PSR-4 file renames.
• Ran the PHPantom test suite and the existing PHP basic/diagnostics suites locally; all passed.