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/this/nested_closure.wren

@@ -13,4 +13,4 @@ class Foo {
 }
 
 var closure = (new Foo).getClosure
-io.write(closure.call.call.call) // expect: Foo
+IO.write(closure.call.call.call) // expect: Foo