Make interpolation of text-only tags more strict (e.g., “<textarea>”).

[theengineear]

The textarea tag has different expectations from other elements. While it’s sensible to allow interpolation of the this element (e.g., to set the default value for this element as a form control) — being overly-lenient actually makes the interface worse. For example, we currently allow:

html`
  <textarea>
    ${someValue}
  </textarea>
`

…but, did the author mean the string \n ${someValue}\n ? Or, did they really mean just ${someValue}? Who’s to say…

It would be better if we instead threw an error in this case and required that integrators strictly write the template like this:

html`
  <textarea>${someValue}</textarea>
`

No room for interpretation 👌