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/close_over_later_variable.wren

@@ -7,7 +7,7 @@ Fn.new {
   var a = "a"
   var b = "b"
   Fn.new {
-    IO.print(b) // expect: b
-    IO.print(a) // expect: a
+    System.print(b) // expect: b
+    System.print(a) // expect: a
   }.call()
 }.call()