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/this/nested_class.wren

@@ -1,13 +1,13 @@
 class Outer {
   method {
-    io.write(this.toString) // expect: Outer
+    IO.write(this.toString) // expect: Outer
 
     fn {
-      io.write(this.toString) // expect: Outer
+      IO.write(this.toString) // expect: Outer
 
       class Inner {
         method {
-          io.write(this.toString) // expect: Inner
+          IO.write(this.toString) // expect: Inner
         }
         toString { return "Inner" }
       }