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/use_before_set.wren

@@ -1,5 +1,5 @@
 class Foo {
-  write { io.write(_field) } // Compile a use of the field...
+  write { IO.write(_field) } // Compile a use of the field...
   init { _field = "value" }  // ...before an assignment to it.
 }