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/variable/many_locals.wren

@@ -255,5 +255,5 @@
   var a253 = a252
   var a254 = a253
   var a255 = a254
-  io.write(a255) // expect: value
+  IO.write(a255) // expect: value
 }