feat: Add a method to Arm64InsnDetail to expose the post_index flag

[PandasAreBears]

The post_index flag was added to cs_arm64 in this commit: f8ae229

This change exposes the value of the flag in Arm64InsnDetail.

Thanks

[tmfink]

I like the change, but could you add the same change to capstone-rs/src/arch/arm.rs?
It would also be great if you could add a unit test for the new methods.

[PandasAreBears]

I've added a test which asserts the correctness of the writeback and post_index fields. It wasn't clear how to cleanly slot it into the existing test_arch_mode_endian_insns pattern. There probably a nicer way of writing this test but I don't have any more time today.