Fix template parts inside comments

[goto-bus-stop]

Comment state is preserved when entering a template ${embed} part,
so you can do things like

html`
  <div>
    <!-- ${someElementThatIWantHidden()} -->
    ${someElementThatIWantShown()}
  </div>
`

instead of the somewhat clumsy

html`
  <div>
    ${''/*
      someElementThatIWantHidden()
    */}
  </div>
`

Of course, in the first situation, someElementThatIWantHidden() is still evaluated, just not inserted.

With opts.comments = true, the embedded parts are stringified and concatenated.