Add Model type to Cadl.Reflection

[pshao25]

We have such decorator to be added:

extern dec exclude(target: Model);

However, Model is not in the Cadl.Reflection

[github-actions]

Changes in this PR will be published to the following url to try(check status of Cadl Pull Request Try It pipeline for publish status):
Playground: https://cadlplayground.z22.web.core.windows.net/prs/1524/

Website: https://cadlwebsite.z1.web.core.windows.net/prs/1524/

[allenjzhang]

@bterlson, @timotheeguerin, @nguerrera, I had an earlier proposal like this, but Tim disagreed and mentioned that object implies model. I don't know if this is a TS convention, but I feel having model is clearer.

BTW, if this change is approved, we need to do a pass on existing libraries to update the decorators.

[nguerrera]

One possible solution would be

namepsace Cadl.Reflection {
   alias Model = object;
}

[nguerrera]

Also, just a note that I don't think the PR as is will achieve the desired result. I think at a minimum, ReflectionNameToKind needs to be updated, and this would need tests. But don't work on this until we have a design approved.

[nguerrera]

I recommend closing this and opening a design issue.

[nguerrera]

See above

[pshao25]

Thanks all. I create issue at #1536. And close this PR.