`buffer` takes either a tab index or a string and focuses the specified
tab. The index can be of the form [0-9]+ which will switch to the
relevant tab in the current window or [0-9]+/[0-9]+ (that is
win_id/index) which will focus the specified window before switching
tabs. If a string is passed the list of open tabs across all windows is
sorted based on title and url (just like in the completion widget) and
the top result is selected.