Decorator and function typing

[nguerrera]

Filing a design issue to track the design needed to be able to do Signature Help: https://github.com/Azure/cadl-azure/issues/614

[nguerrera]

Draft propsoal with two alternatives: https://gist.github.com/nguerrera/626d6a9d65448a9d5cf38e9887fcc1d3

[nguerrera]

I will leave this open to track the design for typing. We have split off #533 as an implementation issue to deliver the direct-from-ts experience regardless of whether or not we use it for typing.

[tjprescott]

This was brought up again at the ARB review on 8/11/2022.

[timotheeguerin]

Updated proposal https://gist.github.com/timotheeguerin/4d01caea2bbd85f96658007fd82daa2e

[timotheeguerin]

Created new issues for this
Overall decorator proposal: #996

• extern decorator implementation extern decorators #994
• Signature help Add signature help for decorators #997