Provide options for flexible line height, fitting text vertically to the container

[BennyHinrichs]

I couldn't find a short name for the spec, so let me know if there is one.

This change would be useful both for contracting things so they don't exceed a parent height and spreading them out to fit the parent height. I don't know what the best way to go about doing this would be. Here are some ideas:

• Add either auto or flex keyword to line-height that will cause the text to vertically fit the container it's in, whether it require a contraction or expansion.

• Make minmax() accessible for line-height.

• Implement a function similar to fit-content().

[fantasai]

@BennyHinrichs I think what you're talking about is similar to https://www.w3.org/TR/css-flexbox-1/#align-content-property except for line boxes? Is that correct?

If so, the tricky thing is that the lines in a block aren't necessarily the direct children of it: frequently they're descendants of those children. So while the idea is straightforward, actually implementing it isn't...