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/implicit_receiver/instance_methods.wren

@@ -2,15 +2,15 @@ class Foo {
   construct new() {}
 
   getter {
-    IO.print("getter")
+    System.print("getter")
   }
 
   setter=(value) {
-    IO.print("setter")
+    System.print("setter")
   }
 
   method(a) {
-    IO.print("method")
+    System.print("method")
   }
 
   test {