feat: implement foreign object finalization and expose wrenCollectGarbage API

Add finalizer invocation for foreign objects during garbage collection by calling wrenFinalizeForeign in wrenFreeObj. Expose wrenCollectGarbage as a public API function and update internal calls accordingly. Ensure the "<finalize>" symbol is always registered in the symbol table even when no finalizer is provided. Add test fixtures for Resource class with finalizer and Api.gc/Api.finalized methods to verify finalization behavior across multiple GC cycles.

src/include/wren.h

@@ -167,6 +167,9 @@ WrenVM* wrenNewVM(WrenConfiguration* configuration);
 // call to [wrenNewVM].
 void wrenFreeVM(WrenVM* vm);
 
+// Immediately run the garbage collector to free unused memory.
+void wrenCollectGarbage(WrenVM* vm);
+
 // Runs [source], a string of Wren source code in a new fiber in [vm].
 //
 // [sourcePath] is a string describing where [source] was located, for use in