Add "Subhead" as option in Text Area components Paragraphs menu

[bcrisler]

Need an additional styling option under the Text Area component's Paragraphs menu: Subhead.

"dcf-d-block dcf-subhead dcf-mb-1 unl-dark-gray"

[skoolbus39]

If possible, subhead should pair with a heading and be wrapped in a <header>, like so:

Subhead before heading:

<header>
  <p class="unl-light-gray dcf-subhead dcf-bold">Subhead Goes Here</p>
  <h2 class="dcf-mt-0">Tortor Dolor Euismod Cursus Venenatis</h2>
</header>

Subhead after heading:

<header>
  <h2>Tortor Dolor Euismod Cursus Venenatis</h2>
  <p class="unl-light-gray dcf-subhead dcf-bold dcf-mb-5">Subhead Goes Here</p>
</header>