Missing Required Indicator on prop form elements

@bnjmnm is the best suited person to triage this.

I'm not sure what to triage past the metadata that's already been set on the issue.
However, it's a simple enough change that there's an MR to take care of this. We could add a bunch of "needs design"-esqe tags to do this thoroughly but I'm partial to getting this out there and allowing the response to these changes inform any changes to the implementation.

I gave !475 a test run, recompiled the UI, flushed some caches, and refreshed. Lo and behold, working required indicators.

This looks great to me, marking RTBC, as the accessibility concern is alleviated by this, even if a follow-up design task needs to happen.

#5++

Should we also be changing FormElementLabel to emit something visually hidden for screen readers too?

— @larowlan

+1

Isn't the required attribute the equivalent signal for non-visual users? It's announced by all major A/T tools.

Adding another signal is probably redundant IMO.

I just checked /node/add/article in Drupal core and saw:

So let's let @bnjmnm have the final say — he's an Accessibility maintainer in Drupal core.