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/setter/static.wren

@@ -1,6 +1,6 @@
 class Foo {
   static bar = value {
-    io.write(value)
+    IO.write(value)
   }
 }