Experimentally add VT to \s definition

Experimentally add VT to \s definition

This commit is the minimal necessary to get \s to match the vertical
tab.  It is being done early in the 5.17 series in order to see what
repercussions there might be from doing this.

It may well be that we decide that this change will require a 'use
feature' to activate.  In any event there is significant documentation
of the behavior without the VT that this patch does not address at all.

Tom Christiansen asked Larry Wall why \s did not include VT, and
reported that Larry replied that he did not remember, but had no
objections to adding it.