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/assignment/associativity.wren

@@ -4,6 +4,6 @@ var c = "c"
 
 // Assignment is right-associative.
 a = b = c
-io.write(a) // expect: c
-io.write(b) // expect: c
-io.write(c) // expect: c
+IO.write(a) // expect: c
+IO.write(b) // expect: c
+IO.write(c) // expect: c