Add Model type to Cadl.Reflection - #1524

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/

Brian Terlson (@bterlson), Timothee Guerin (@timotheeguerin), Nick Guerrera (@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.

One possible solution would be

namepsace Cadl.Reflection {
   alias Model = object;
}

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.

I recommend closing this and opening a design issue.

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