Support specifying visibility decorator on models

### Clear and concise description of the problem

Currently, the visibility decorator can only be set on properties. It would be nice to allow it to be set on a model so that if all properties for a model should be given a specific visibility, it can be expressed more concisely. The visibility specified on the model would be propagated to the properties.

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [docs](https://typespec.io/docs/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.