Migrate the foreign method argument access API from an argument-oriented naming
scheme to a slot-based one, renaming wrenGetArgumentCount to wrenGetSlotCount,
wrenGetArgumentBool to wrenGetSlotBool, and all other argument accessors
accordingly. Update every call site across the VM, modules (io, timer, meta,
random), and test files (benchmark, foreign_class) to use the new names. The
slot getters now assert the correct type at runtime rather than silently
returning defaults, shifting safety responsibility to the caller for
performance. Also add the IS_LIST type-checking macro to wren_value.h alongside
the existing IS_* macros.
The number of arguments can be derived from the stack pointer difference, eliminating the need to store it separately. Rename the slot pointer to better reflect its purpose as the start of the foreign call's stack region.
Rename the `src/aux/` directory to `src/optional/`, update all include guards, preprocessor defines, and file references from `WREN_AUX_*` to `WREN_OPT_*`, and adjust build system, Xcode project, and code generation scripts accordingly.
