Move tabs as far as possible if index out of range

If a window contains just pinned or just unpinned tabs, `999<` and `999>` (or some other sufficiently large number) move a tab to the first and Iast positions, respectively. Currently, if a window contains a mixture of pinned *and* unpinned tabs, `999<` does not move an unpinned tab, and `999>` does not move a pinned tab.

I consider it to be a bug with the `chrome.tabs.move` API, but this PR works around that.