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/closure/close_over_method_parameter.wren

@@ -3,7 +3,7 @@ var f = null
 class Foo {
   method(param) {
     f = fn {
-      io.write(param)
+      IO.write(param)
     }
   }
 }