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/sequence/count.wren

@@ -10,4 +10,4 @@ class TestSequence is Sequence {
   iteratorValue(iterator) { iterator }
 }
 
-IO.print(TestSequence.new().count) // expect: 10
+System.print(TestSequence.new().count) // expect: 10