Add size-text shorthand

[giuseppeg]

This PR adds a size-text utility that sets width and height to 1em.

This is super useful when making icons and other elements that can then be controlled by setting a font-size on the parent or the element itself.

Relates to #12287

[adamwathan]

Hey thanks for the PR! For now going to say no to this because we don’t include this size anywhere else in the framework, so it’s a bit of a bigger decision and can’t prioritize thinking it through right now. Maybe in the future though 👍🏻

[giuseppeg]

No worries Adam! I submitted this because I use it on every project and have seen others doing the same too but I am sure you have a better overview of what is widely needed and what is not