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/core/object/not.wren

@@ -1,4 +1,4 @@
 class Foo {
   construct new() {}
 }
-IO.print(!Foo.new()) // expect: false
+System.print(!Foo.new()) // expect: false