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/module/inside_block/inside_block.wren

@@ -4,7 +4,7 @@ if (true) {
   import "module" for Module
   // expect: ran module
 
-  IO.print(Module) // expect: from module
+  System.print(Module) // expect: from module
 }
 
-IO.print(Module) // expect: outer
+System.print(Module) // expect: outer