custom elements can't contain slot='...' children

[Rich-Harris]

There are probably a few different ways we need to fix our handling of custom elements (e.g. always using props, instead of attributes, when passing data), this just happens to the be the first one I've run across:

<custom-element>
  <header slot='header'>header header header</header>
</custom-element>

This fails because Svelte thinks the slotted element should belong to a Svelte component, and it can't find one.