Add text-wrap: balance to all headings

[janbrasna]

Description

https://caniuse.com/css-text-wrap-balance is pretty well supported now with no real downside, known bugs or performance issues documented.

• I have documented this change in the design system.
• I have recorded this change in CHANGELOG.md.

Issue

Resolves #910

Testing

Safari 16 vs. 17.5+
Firefox ESR (115) vs. 122+

TODO: interaction with white-space

[stephaniehobson]

@janbrasna Do you remember what the problem is with white-space?

[janbrasna]

@stephaniehobson So the issue is what risk this might pose for downstream consumers, where balancing text IIUC actually competes with white-space property per se (as one is asking for no wrapping and the other is asking for balanced wrapping) — guess I wanted to check a few layouts over at kitsune, fxa, relay et al. how such a change wholesale could interfere with what they have added to the cascade in addition to protocol styling.

(For just bedrock this should be a non-issue, as balancing headings has been around for some time now.)

[stephaniehobson]

Seems like it follows the cascade in Firefox and Chrome so any custom styles they have applied should take president if they're importing their styles after the Protocol one.

I think we're safe to go ahead but I'd add something to the CHANGELOG's migration notes to search for any place where white-space: nowrap is defined and make sure it has a higher specificity.

[stephaniehobson]

It might be a nice change to include with V20 which will have other font updates.