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/method/many_methods.wren

@@ -2022,4 +2022,4 @@ result = result + foo.method996
 result = result + foo.method997
 result = result + foo.method998
 result = result + foo.method999
-IO.print(result) // expect: 1000
+System.print(result) // expect: 1000