fix: Clear virtual text immediately on cursor move

The PR #121 introduced some side-effects which made the plugin feel more laggy overall because of how the virtual text got cleared with a delay. This commit retains the original delay behavior for displaying, while immediately clearing the virtual text on cursor move.

Previous behavior:

https://github.com/f-person/git-blame.nvim/assets/31893391/59f68069-7741-4af3-9857-c130f1c5877b

Current behavior:

https://github.com/f-person/git-blame.nvim/assets/31893391/49c8e320-8e63-480d-bf44-c5a70c725f7b

@bossley9 this doesn't yet switch the default to 0 since it's probably good to 1) reconsider doing that after this change (i would probably still be in favor of that), 2) nicer to do in a separate PR

@jinzhongjia maybe also take a look at this one if you feel like it :)

The effect is great