feat: replace IO class with System class and remove separate IO module

- Delete wren_io.c, wren_io.h, and io.wren source files
- Remove multi-arity print overloads and IO.read/IO.time methods
- Add System class with print, printAll, write, writeAll methods to core.wren
- Update all documentation examples and README to use System.print instead of IO.print

test/language/closure/closed_closure_in_function.wren

@@ -3,7 +3,7 @@ var f = null
 {
   var local = "local"
   f = Fn.new {
-    IO.print(local)
+    System.print(local)
   }
 }