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/field/use_before_set.wren

@@ -1,6 +1,6 @@
 class Foo {
   construct new() {}
-  write { IO.print(_field) } // Compile a use of the field...
+  write { System.print(_field) } // Compile a use of the field...
   init { _field = "value" }  // ...before an assignment to it.
 }