refactor: replace finalizer fiber with raw data pointer callback

Remove the dedicated finalizer fiber from WrenVM and change finalizer functions to accept a void* data pointer instead of a full WrenVM reference. This prevents user code from interacting with the VM during garbage collection, simplifying the GC path and eliminating the need for pre-allocated fiber infrastructure. Update the WrenForeignClassMethods struct, all built-in finalizers (file, test), and the binding/calling logic accordingly.

test/io/file/finalize.wren

@@ -0,0 +1,10 @@
+import "io" for File
+
+// Don't store in a variable.
+File.open("test/io/file/finalize.wren")
+
+System.gc()
+// We can't really test what the finalizer *does* from Wren, since the object
+// is unreachable, but this at least ensures it doesn't crash.
+
+System.print("ok") // expect: ok