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/shadow_closure_with_local.wren

@@ -2,10 +2,10 @@
   var foo = "closure"
   Fn.new {
     {
-      IO.print(foo) // expect: closure
+      System.print(foo) // expect: closure
       var foo = "shadow"
-      IO.print(foo) // expect: shadow
+      System.print(foo) // expect: shadow
     }
-    IO.print(foo) // expect: closure
+    System.print(foo) // expect: closure
   }.call()
 }