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()`.

benchmark/fib.wren

@@ -9,7 +9,7 @@ var fib = fn(n) {
 var start = OS.clock
 var i = 0
 while (i < 5) {
-  io.write(fib.call(28))
+  IO.write(fib.call(28))
   i = i + 1
 }
-io.write("elapsed: " + (OS.clock - start).toString)
+IO.write("elapsed: " + (OS.clock - start).toString)