[css-align][css-inline] Deal with first vs last baseline #210

The CSS Box Alignment spec introduces the ability to choose the last baseline instead of the first for baseline-aligning boxes. This ability needs to also be ported to the vertical-align property (in CSS Inline Layout). Note, the vertical-align property is especially wonky because for inline-tables it uses the first baseline and for inline-blocks it uses the last.

The current syntax in Box Alignment is to contrast baseline (imported from Flexbox) with last-baseline (new in Box Alignment). However, using prefixed keywords would result in a combinatorial explosion if we added it to the alignment-baseline subproperty of vertical-align.

Some solutions to this problem:

1. Explode alignment-baseline.
2. Use last and baseline as separated keywords in align/justify-self/content as well as alignment-baseline.
3. Use last-baseline in align/justify-self/content and last baseline in alignment-baseline.
4. Introduce a new property to choose first vs last for vertical-align, and have last-baseline decompose to last in that property plus baseline for alignment-baseline.
5. Introduce first-baseline and last-baseline to alignment-baseline(to match Align), but also allow first and last space-separated prefixes for all values of alignment-baseline(to avoid explosion). (This means that both first-baseline and first baseline would be valid, but eh?)
6. Something else?