Intrinsic sizing keywords (min/max/fit-content) for width/height

[nicoburns]

What problem does this solve or what need does it fill?

These keywords allow users to opt-in to sizing behaviour that Taffy already supports but can currently only be accessed implicitly or via grid-template sizing functions.

What solution would you like?

• Add MinContent, MaxContent, and FitContent variants to Dimension.
• Implement the sizing logic in the Flexbox, Grid, and Block layout algorithms by overriding the AvailableSpace provided to child nodes if their width/height is one of these keywords.
• We will need to be careful that this will remove the invariant that available_space == parent_size

Additional context

A bunch of Flexbox WPT tests rely on these keywords, so if we want accurate test results then we need to implement them!