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/scope_reuse_in_different_blocks.wren

@@ -1,9 +1,9 @@
 {
   var a = "first"
-  io.write(a) // expect: first
+  IO.write(a) // expect: first
 }
 
 {
   var a = "second"
-  io.write(a) // expect: second
+  IO.write(a) // expect: second
 }