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

example/mandelbrot.wren

@@ -29,8 +29,8 @@ for (yPixel in 0...24) {
       }
     }
 
-    IO.write(pixel)
+    System.write(pixel)
   }
 
-  IO.print("")
-}
+  System.print()
+}