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/number/literals.wren

@@ -1,10 +1,10 @@
-io.write(123)     // expect: 123
-io.write(987654)  // expect: 987654
-io.write(0)       // expect: 0
-io.write(-0)      // expect: -0
+IO.write(123)     // expect: 123
+IO.write(987654)  // expect: 987654
+IO.write(0)       // expect: 0
+IO.write(-0)      // expect: -0
 
-io.write(123.456) // expect: 123.456
-io.write(-0.001)  // expect: -0.001
+IO.write(123.456) // expect: 123.456
+IO.write(-0.001)  // expect: -0.001
 
 // TODO: Hex? Scientific notation?
 // TODO: Literals at and beyond numeric limits.