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/range/join.wren

@@ -1,4 +1,4 @@
 var a = 1..3
 
-IO.print(a.join) // expect: 123
-IO.print(a.join(", ")) // expect: 1, 2, 3
+System.print(a.join) // expect: 123
+System.print(a.join(", ")) // expect: 1, 2, 3