Error if element name is capitalised

[Rich-Harris]

I keep writing <SomeNewComponent/> and forgetting to import SomeNewComponent and add it to components. I could save myself some time if the compiler printed a warning (or maybe if it errored?) on encountering a capitalised tag that doesn't correspond to a known component.

[Conduitry]

I like this idea, it seems like a tidy solution to this problem. I think the main bump I see here is that making this an outright error would probably be a breaking change. A sane and likely desirable change, but probably a breaking one. Not sure whether that influences how you want this handled.

[Rich-Harris]

Good point. Let's make it a warning for now, with a TODO comment to upgrade it to an error in v2