[Docs] Add guide for debugging PHP in WebAssembly with Xdebug

[fellyph]

This pull request adds new documentation for debugging PHP code with Xdebug using the PHP WASM CLI. The documentation provides step-by-step instructions for configuring and running Xdebug with popular IDEs, making it easier for developers to debug PHP applications in a WebAssembly environment.

New documentation:

• Added a comprehensive guide in 07-xdebug/03-xdebug-on-php-wasm.md explaining how to debug PHP code with Xdebug in the PHP WASM CLI, including prerequisites, IDE configuration steps for VS Code/Cursor and PhpStorm, usage instructions, and command reference.