Consider adding separators to thread nav buttons (Follow-up from "Fix radii of MR thread nav buttons")
The following discussion from !205942 should be addressed: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/205942#note_2770741129
> Related, this whole control is very similar to a [label button](https://design-gitlab-com-gitlab-org-gitlab-services-f6beaf2fb7fd4cad76.gitlab.io/storybook/?path=/story/base-button--label-button), which has separation between the buttons to clarify the label and the buttons. Trying to adapt that to here, that would be:
>
> | Label button example | All borders | Border between label and button |
> |----------------------|-------------|---------------------------------|
> |  |  |  |
>
> It's strange that we are not applying this component here, as we do in other places, like the commits views.
issue