Add items-last-baseline

[emctague]

last baseline aligns items via their last, rather than first, baseline. This would be a useful addition to the items- classes:

.items-last-baseline {
  align-items: last baseline;
}

There was an old discussion about this but it appears to have petered out without being acknowledged. It also suggested adding a first-baseline, but as far as I can tell, first baseline is equivalent to baseline.