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/closure/nested_closure.wren

@@ -7,9 +7,9 @@ Fn.new {
     Fn.new {
       var c = "c"
       f = Fn.new {
-        IO.print(a)
-        IO.print(b)
-        IO.print(c)
+        System.print(a)
+        System.print(b)
+        System.print(c)
       }
     }.call()
   }.call()