feat: add wrenGetVariable API to load top-level module variable into a slot

Implement wrenGetVariable function in wren_vm.c that looks up a top-level variable by name in a specified module and stores it in the given slot. Includes validation for non-null module/name parameters and slot bounds, module resolution via getModule, variable lookup through symbol table, and slot assignment. Adds corresponding declaration to wren.h public header. Introduces comprehensive test suite in test/api/get_variable.* covering scenarios: retrieving variable before definition (null), after definition, after reassignment, using a non-zero slot, and accessing a variable from another module. Registers the test bindings in test/api/main.c and updates Xcode project file to include the new source.

src/include/wren.h

@@ -384,4 +384,9 @@ void wrenSetSlotValue(WrenVM* vm, int slot, WrenValue* value);
 // an element, use `-1` for the index.
 void wrenInsertInList(WrenVM* vm, int listSlot, int index, int elementSlot);
 
+// Looks up the top level variable with [name] in [module] and stores it in
+// [slot].
+void wrenGetVariable(WrenVM* vm, const char* module, const char* name,
+                     int slot);
+
 #endif