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/string/literals.wren

@@ -1,2 +1,2 @@
-io.write("".count)   // expect: 0
-io.write("a string") // expect: a string
+IO.write("".count)   // expect: 0
+IO.write("a string") // expect: a string