How can I add arbitrary placeholder text to form fields? I'm converting an existing site (built by someone else) from Drupal 7 to Drupal 8.

The old site uses the Field Placeholder module to add placeholders to form fields. However, there isn't yet a Drupal 8 version of this module.

I came across the Form Placeholder module, which does have a Drupal 8 version – but it won't work for my case. (Most significantly, it only allows you to use the field label as the placeholder – you can't enter an arbitrary placeholder that's different from the label.)

Is there another option I can consider?

(I'm aware that I could hack something together with jQuery. For various reasons, I'd prefer not to do that.)
It seems that this functionality is now in the Drupal core – placeholders can be added from the widget settings on the content type's form display page.