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/implicit_receiver/locals_shadow_getter.wren

@@ -4,15 +4,15 @@ class Foo {
   bar { "getter" }
 
   test {
-    IO.print(bar) // expect: getter
+    System.print(bar) // expect: getter
 
     {
-      IO.print(bar) // expect: getter
+      System.print(bar) // expect: getter
       var bar = "local"
-      IO.print(bar) // expect: local
+      System.print(bar) // expect: local
     }
 
-    IO.print(bar) // expect: getter
+    System.print(bar) // expect: getter
   }
 }