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/method/static.wren

@@ -6,7 +6,7 @@ class Foo {
   static bar(arg) { return "on metaclass " + arg }
 }
 
-io.write((new Foo).bar)        // expect: on instance
-io.write(Foo.bar)              // expect: on metaclass
-io.write((new Foo).bar("arg")) // expect: on instance arg
-io.write(Foo.bar("arg"))       // expect: on metaclass arg
+IO.write((new Foo).bar)        // expect: on instance
+IO.write(Foo.bar)              // expect: on metaclass
+IO.write((new Foo).bar("arg")) // expect: on instance arg
+IO.write(Foo.bar("arg"))       // expect: on metaclass arg