Can't nest facon invocations

[quat1024]

#5 says it allows nestable facon invocations, but it doesn't seem to work :(

This

const foo = f`<span>hello</span>`;
const bar = f`<h1>${foo}</h1>`;
document.body.appendChild(bar);

results in this being written to the document

🙁

Still not too familiar with the JS world, but DocumentFragment (the contents of template.content) is not an instance of HTMLElement according to MDN so I think the code as-written is behaving "correctly". Personally I changed instanceof HTMLElement to instanceof Node and it seems to work, but i haven't tested it too well (idk what happens if you try to splice in multiple children, for example)