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/conditional/short_circuit.wren

@@ -1,2 +1,2 @@
-true ? IO.print("ok") : IO.print("no") // expect: ok
-false ? IO.print("no") : IO.print("ok") // expect: ok
+true ? System.print("ok") : System.print("no") // expect: ok
+false ? System.print("no") : System.print("ok") // expect: ok