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

[fantasai]

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?

[fantasai]

Discussion on the WG call rejected 1 & 3, seem to have a slight preference for 4. We should probably investigate whether/how 'alignment-baseline' is applied to non-inlines, since that might make a difference here as well.

[FremyCompany]

Just said on today's call I had a preference for a "baseline-choice: first / last".

[FremyCompany]

Fantasai pointed out we want an auto value to reflect tables choosing the first and inline-block the last by default.

[tabatkins]

Resolution was to change the alignment properties to have [ first | last ]? baseline.

[fantasai]

Fix checked in 4f6d5ad