Error on invalid tag names

[Rich-Harris]

Describe the problem

As of #12821, we error on invalid component names. We don't have any similar validation for element names.

In Svelte 4, something like <yes[no]> is a runtime error, because you can't do this:

document.createElement('yes[no]');

Because we use template cloning in Svelte 5, we don't hit that constraint — it just lets you create an obviously-incorrect element. We should have some validation here

Describe the proposed solution

Either replace the 'invalid component name' error with a more generic 'invalid tag name' error, or have some way of determining whether a tag name is supposed to be a component or an element before we've been able to determine whether it's valid

Importance

nice to have