feat: deprecate enum val

[brainwo]

Both State.val and Rating.val are unnecessary. Dart already has Enum.index, this commit marks the unnecessary properties to be deprecated in future major version release.

This changes won't affect users of this library.

[brainwo]

@sonbui00 do you have any thoughts on this?
I would argue removing val is important to make the code cleaner, but I'm still not sure whether it should be deprecated first or just remove it entirely. I don't think val would be used anywhere outside of this library internal logic.

If you prefer to keep val, a getter can be a solution:

 enum Rating {
-  again(1),
-  hard(2),
-  good(3),
-  easy(4);
+  again,
+  hard,
+  good,
+  easy;
 
-  const Rating(this.val);
+  const Rating();
 
-  final int val;
+  int get val => index + 1;
 }

[sonbui00]

@brainwo

If you prefer to keep val, a getter can be a solution:

I think I don't use index because the Python code use the mapping

https://github.com/open-spaced-repetition/py-fsrs/blob/a200a78858bca48e782d6cdf25c8086a3ee290cf/src/fsrs/models.py#L15

I also agree that the enum should not use the val. I added the val to match with logic from Python. Let's me check if the val have any meaning in the process. As I remeber they use that number in somewhere, maybe on optimizer