Moving flexbox behind a feature flag

[Weibye]

Objective

Why did you make this PR?

Fix #298

Feedback wanted

• Does it make sense that Display::None is the default value for the node?

[alice-i-cecile]

Good, I really like feature flagging this.

[nicoburns]

Mostly looks good, but I have a few suggestions. Additionally conflicts with main should be resolved (also clippy lints, but it looks like those have been fixed on main :))

[nicoburns]

I've pushed a couple of changes:

• justify_items and justify_self are now gated behind the grid feature only. They do not apply to Flexbox.
• I renamed a job back to it's original name. This is required to make the github actions pass. @alice-i-cecile I'm not sure if there's some setting we change (perhaps temporarily) to enable the names of github actions jobs to be changed? I think the "required actions" use the names from the main branch, but you can't commit to main without a PR, and you can't get a PR to pass CI with renamed jobs! Not urgent, but would be nice to sort out the names at some point.

[Weibye]

• justify_items and justify_self are now gated behind the grid feature only. They do not apply to Flexbox.

Right! It's been a while and it's challenging to keep track of all the similar names 😅