refactor: convert IO from singleton instance to static class with metaclass native methods

Replace the global `io` singleton object with a static `IO` class, moving the `write` native from the instance to the metaclass. Update all benchmark, example, and test files to use `IO.write()` instead of `io.write()`.

test/field/default_to_null.wren

@@ -1,5 +1,5 @@
 class Foo {
-  write { io.write(_field) }
+  write { IO.write(_field) }
 }
 
 (new Foo).write // expect: null