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/add.wren

@@ -1,10 +1,10 @@
 var a = []
 a.add(1)
-io.write(a) // expect: [1]
+IO.write(a) // expect: [1]
 a.add(2)
-io.write(a) // expect: [1, 2]
+IO.write(a) // expect: [1, 2]
 a.add(3)
-io.write(a) // expect: [1, 2, 3]
+IO.write(a) // expect: [1, 2, 3]
 
 // Returns added element.
-io.write(a.add(4)) // expect: 4
+IO.write(a.add(4)) // expect: 4