runtime(menu): define shortcut for File->Open Tab (vim/vim#12895)
Seems missing as noted by Antonio Giovanni Colombo. So add it and use
the 'T' as shortcut, which does not seem to be used in the File dialog.
Verified on Windows.
e059fae100
Co-authored-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Antonio Giovanni Colombo <azc100@gmail.com>