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/limit/many_constants.wren

@@ -65534,7 +65534,7 @@ var f = Fn.new {
   65533
   65534
   65535
-  IO.print(65536)
+  System.print(65536)
 }
 
 f.call() // expect: 65536