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/close_over_function_parameter.wren

@@ -2,7 +2,7 @@ var f = null
 
 Fn.new {|param|
   f = Fn.new {
-    IO.print(param)
+    System.print(param)
   }
 }.call("param")