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/break/closure_in_for.wren

@@ -1,7 +1,7 @@
 var f
 for (i in [1, 2, 3]) {
   var j = 4
-  f = Fn.new { IO.print(i + j) }
+  f = Fn.new { System.print(i + j) }
   break
 }