feat(layouts): edit panes

[imsnif]

This adds edit panes in layouts. These are panes that include a path to a file that will be opened with the user's default EDITOR (or scrollback_editor config item).

eg.

layout {
    pane split_direction="vertical" {
        pane edit="./git_diff_side_a"
        pane edit="./git_diff_side_b"
    }
}

EDIT: Documentation added to #1759 where we keep that sort of thing for now.