Introduce a new public C API function wrenGetSlotType() that returns the
underlying representation type (bool, num, foreign, list, null, string,
or unknown) of the object stored in a given VM slot. This allows C
extensions to inspect slot contents without needing to call individual
type-checking functions or handle errors from mismatched accessors.
The implementation in wren_vm.c uses the existing IS_* macros to
determine the type and returns the corresponding WrenType enum value.
A new WrenType enum is added to wren.h with WREN_TYPE_BOOL,
WREN_TYPE_NUM, WREN_TYPE_FOREIGN, WREN_TYPE_LIST, WREN_TYPE_NULL,
WREN_TYPE_STRING, and WREN_TYPE_UNKNOWN.
Test coverage is added in test/api/slots.c with a slotTypes() foreign
method that verifies all seven type values, and a ForeignType foreign
class is introduced in slots.wren to provide a foreign object for
testing WREN_TYPE_FOREIGN. The test harness in main.c is updated to
support binding foreign class allocate methods via slotsBindClass().
Bob Nystrom
fe3997f825