feat: add wrenGetSlotType API to query low-level type of object in a slot

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().

src/include/wren.h

@@ -162,12 +162,30 @@ typedef struct
   int heapGrowthPercent;
 } WrenConfiguration;
 
-typedef enum {
+typedef enum
+{
   WREN_RESULT_SUCCESS,
   WREN_RESULT_COMPILE_ERROR,
   WREN_RESULT_RUNTIME_ERROR
 } WrenInterpretResult;
 
+// The type of an object stored in a slot.
+//
+// This is not necessarily the object's *class*, but instead its low level
+// representation type.
+typedef enum
+{
+  WREN_TYPE_BOOL,
+  WREN_TYPE_NUM,
+  WREN_TYPE_FOREIGN,
+  WREN_TYPE_LIST,
+  WREN_TYPE_NULL,
+  WREN_TYPE_STRING,
+  
+  // The object is of a type that isn't accessible by the C API.
+  WREN_TYPE_UNKNOWN
+} WrenType;
+
 // Initializes [configuration] with all of its default values.
 //
 // Call this before setting the particular fields you care about.
@@ -263,6 +281,9 @@ int wrenGetSlotCount(WrenVM* vm);
 // It is an error to call this from a finalizer.
 void wrenEnsureSlots(WrenVM* vm, int numSlots);
 
+// Gets the type of the object in [slot].
+WrenType wrenGetSlotType(WrenVM* vm, int slot);
+
 // Reads a boolean value from [slot].
 //
 // It is an error to call this if the slot does not contain a boolean value.