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/list/count.wren

@@ -1,3 +1,3 @@
-io.write([].count)           // expect: 0
-io.write([1].count)          // expect: 1
-io.write([1, 2, 3, 4].count) // expect: 4
+IO.write([].count)           // expect: 0
+IO.write([1].count)          // expect: 1
+IO.write([1, 2, 3, 4].count) // expect: 4