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/syntax.wren

@@ -2,10 +2,10 @@
 var a = "a"
 var b = "b"
 a = b = "chain"
-io.write(a) // expect: chain
-io.write(a) // expect: chain
+IO.write(a) // expect: chain
+IO.write(a) // expect: chain
 
 // Assignment on RHS of variable.
 var c = a = "var"
-io.write(a) // expect: var
-io.write(c) // expect: var
+IO.write(a) // expect: var
+IO.write(c) // expect: var