Upgrade to Taffy 0.6

[nicoburns]

Objective

• Keep Taffy version up to date

Taffy 0.6 doesn't include a huge amount relevant to Bevy. But it does:

• Add the box_sizing style
• Expose the computed margin in layout
• Traitifies the Style struct, which opens up the possibility of using Bevy's Style struct directly (although Bevy currently does some style resolution at conversion time which would no longer be cached if it was used directly).
• Have a few bug fixes in the layout algorithms

Solution

• Upgrade Taffy to 0.6.0

Testing

• I've run the grid example. All looks good.
• More testing is probably warranted. We have had regressions from Taffy upgrades before
• Having said that, most of the algorithm changes this cycle were driven by fixing WPT tests run through the new Servo integration. So they're possibly less likely than usual to cause regressions.

Breaking changes

The only "breaking" change is adding a field to Style. Probably doesn't bear mentioning?

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[ickshonpe]

Looks good besides the comments nits.

Adding the computed margin values would let me add support for margin-box to #15561.

[nicoburns]

Nits addressed