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/nonlocal/mutual_recursion.wren

@@ -8,5 +8,5 @@ class Bar {
   static foo { Foo.new() }
 }
 
-IO.print(Foo.bar) // expect: instance of Bar
-IO.print(Bar.foo) // expect: instance of Foo
+System.print(Foo.bar) // expect: instance of Bar
+System.print(Bar.foo) // expect: instance of Foo