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/class/field_in_foreign_class.wren

@@ -1,7 +1,7 @@
 foreign class Foo {
   bar {
     // Can't read a field.
-    IO.print(_bar) // expect error
+    System.print(_bar) // expect error
 
     // Or write one.
     _bar = "value" // expect error