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/nonlocal/undefined.wren

@@ -1,6 +1,6 @@
 var fn = Fn.new {
-  IO.print(Foo)
-  IO.print(Bar)
+  System.print(Foo)
+  System.print(Bar)
 }
 
 // expect error line 6