add `strictUnusedTemplate` to template type checker about angular HOT 2 CLOSED

This is a little difficult to conceive, since it's hard to tell if the <ng-template> is used or not. It might be in the projected content of another component, where it could be queried by the name foo or simply TemplateRef. The compiler could theoretically know which content would be projected and thus subject to querying, but that's getting very nuanced. I think this is a good candidate for a lint error.

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I thought it was possible, as my intellij IDEA grays out unused templates:

But I admit it's not 100% accurate.

I didn't even know that child components could yoink templates from projected content. That of course complicates this inspection. Will see if I suggest this at angular-eslint instead.

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