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/variable/many_nonsimultaneous_locals.wren

@@ -259,7 +259,7 @@
     var a253 = a252
     var a254 = a253
     var a255 = a254
-    io.write(a255) // expect: value a
+    IO.write(a255) // expect: value a
   }
 
   {
@@ -519,6 +519,6 @@
     var b253 = b252
     var b254 = b253
     var b255 = b254
-    io.write(b255) // expect: value b
+    IO.write(b255) // expect: value b
   }
 }