Fallback for gitlab.move_to_discussion_tree_from_diagnostic

[jakubbortlik]

Feature Description

In my gitlab.nvim config I've mapped both gitlab.move_to_discussion_tree_from_diagnostic and jump_to_reviewer to the same key (a) and I use it to quickly jump between the discussion tree and the reviewer. This workflow is interrupted when jumping to the reviewer for a resolved thread with settings.discussion_signs.skip_resolved_discussion = true. Then I just get the warning "No diagnostics for this line".

I'd like the gitlab.move_to_discussion_tree_from_diagnostic function to allow a fallback for such cases. Instead of just showing a warning, I'd like focus to be brought to the last position in the discussion tree.

I imagine it should be fairly easy to implement as an opt-in feature, so the default behaviour would not change. I can open a PR after the other pending PR are sorted out.