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/number/plus.wren

@@ -1,3 +1,3 @@
-IO.print(1 + 2) // expect: 3
-IO.print(12.34 + 0.13) // expect: 12.47
-IO.print(3 + 5 + 2) // expect: 10
+System.print(1 + 2) // expect: 3
+System.print(12.34 + 0.13) // expect: 12.47
+System.print(3 + 5 + 2) // expect: 10