Use innerHTML in more places

[Rich-Harris]

Moving this from #3341. In a case like this...

<script>
  const answer = 42;
</script>

<div>
  <span class="a/{answer}"/>
</div>

...Svelte should recognise that answer cannot change, and so it's ok to use innerHTML:

-span = element("span");
-attr(span, "class", span_class_value = "a/" + answer);
+div.innerHTML = `<span class="a/${answer}"></span>`;
// ...
-append(div0, span);

[Conduitry]

I believe this was fixed back with #3808 and some follow-up PRs.