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/string/bytes.wren

@@ -3,4 +3,4 @@
 // Chars: sø mé ஃ  thî ng
 var s = "søméஃthîng"
 
-IO.print(s.bytes is StringByteSequence) // expect: true
+System.print(s.bytes is StringByteSequence) // expect: true