Add hover/autocomplete for intrinsic functions

Currently intrinsic functions (Ref, GetAtt, etc.) are supported but we don't get additional language server details from them. 

As an example we don't get autocomplete of a Ref to cover parameters and resources.  Additionally when I hover on an intrinsic function I want details about I'm referring to.

- [x] Ref
- [ ] GetAtt
- [ ] FindInMap
- [ ] If