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/variable/many_nonsimultaneous_locals.wren

@@ -259,7 +259,7 @@
     var a253 = a252
     var a254 = a253
     var a255 = a254
-    IO.print(a255) // expect: value a
+    System.print(a255) // expect: value a
   }
 
   {
@@ -519,6 +519,6 @@
     var b253 = b252
     var b254 = b253
     var b255 = b254
-    IO.print(b255) // expect: value b
+    System.print(b255) // expect: value b
   }
 }