Grid: baseline alignment support

[nicoburns]

Objective

Support baseline alignment in the CSS Grid Algorithm.

This implements both:

• Computing the first baseline of a grid container
• Baseline aligning items within grid rows

There is plenty more that could be supported such as vertical baselines and last baselines. But this brings us up to parity with what is implemented for flexbox.

[alice-i-cecile]

Clear and well-tested, like usual.