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/class/new.wren

@@ -1,6 +1,6 @@
 class Foo {}
 
 var foo = new Foo
-io.write(foo is Foo) // expect: true
+IO.write(foo is Foo) // expect: true
 
 // TODO: Test precedence and grammar of what follows "new".