adding functions for interacting with execution context

[brettmc]

This pull request adds new FFI (Foreign Function Interface) wrappers and Rust-side helpers for working with PHP internals, particularly around function and execution context introspection. The changes improve the ability to interact with PHP's call stack, function flags, and class scopes from Rust code, and also provide more ergonomic and safe APIs.

New PHP internals FFI wrappers:

• Added phper_zval_undef to set a zval to UNDEF and exposed it to Rust.
• Added wrappers for call stack introspection: phper_get_called_scope, phper_zend_call_num_args, phper_zend_set_call_num_args, phper_zend_call_info, phper_zend_add_call_flag, and phper_zend_call_may_have_undef. Also added helpers for success/failure constants. [1] [2]

Rust-side enhancements:

• Added is_static method to ZFunc to check if a function is static using ZEND_ACC_STATIC.
• Improved ExecuteData::get_this and get_this_mut to use safe pointer conversion and added get_called_scope to retrieve the called class scope as an optional ClassEntry.
• Updated module imports in phper/src/values.rs to include ClassEntry for new functionality.