feat(explorer): Virtual line spacer between Conflict, Staged and Unstaged groups

[kikefdezl]

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Why?

When the explorer displays dozens of files, I often lose track of the Staged and Unstaged groups. I end up staging/unstaging files by accident because of this.

A single line spacer makes it much more clear where the unstaged files stop and the staged files start.

What was done

I introduced a margin_top attribute to the Node. When rendering the explorer, if margin_top = n, we render n virtual lines above this group.

For the Staged Group Node, I set margin_top to be 1 if it's visible.

I also added some type annotations for the Node tables.

Configuration & Opt-in vs Opt-out

This PR makes the change without exposing any configuration, since I believe it is a sensible default. If you would prefer to expose this via configuration and/or make it opt-in, I would be glad to update the PR with those changes.

[h0adp0re]

What if only "Staged Changes" is present?

[kikefdezl]

What if only "Staged Changes" is present?

If the Unstaged group visibility is toggled off, the Staged group will stick to the top because we only add the margin to a group if there are any nodes above.

margin_top = (#tree_nodes > 0) and 1 or nil,