warn on invalid angle bracket invocation

[Deleted user]

Orignally opened in emberjs/ember.js#18178.

As our team has started using angle bracket invocation for components, a few "invalid" patterns have started:

<My-card @list={{this.list}} />
<My-other-thing>
</My-other-thing>

I consider this bug in the same group as emberjs/ember.js#17998; developer experience.
As a developer, I would like to see an error about unexpected capitalization when using angle brackets.