Implement box-sizing: content-box

[nicoburns]

Objective

• Fixes Support box-sizing: content-box #289
• Closes Document that Taffy uses box-sizing: border-box #417

Context

• The web has two box sizing modes that modify how size styles affect a node's size: box-sizing: border-box and box-sizing: content-box. Taffy has previously assumed box-sizing: border-box, and while that's generally a good default, box-sizing: content-box is actually the default on the web, and is thus crucial for web compatibility.
• This is breaking change (due to a new style being added). So we may want to put out a patch release before merging.

[nicoburns]

Hmm... this seems to be causing benchmark regressions in the ~15% range for CSS Grid. Will need to look into this.