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/multiple.wren

@@ -10,11 +10,11 @@ class Foo {
   }
 
   write {
-    IO.print(_a)
-    IO.print(_b)
-    IO.print(_c)
-    IO.print(_d)
-    IO.print(_e)
+    System.print(_a)
+    System.print(_b)
+    System.print(_c)
+    System.print(_d)
+    System.print(_e)
   }
 }