pane: Do not autopin new item created as a neighbour of pinned tab (#18072)

When I used editor::NewFile or ProjectSearch from a pinned tab, the
resulting new tab would be pinned (and the last pinned tab would be
pushed off). This PR fixes it by always storing new tabs outside of the
pinned area if there's no destination index for the new tab.

Release Notes:

- Fixed tab bar not preserving pinned tab state when an editor::NewFile
action is executed.