Implement values grammar

[Version 0.57.0](https://github.com/microsoft/typespec/releases/tag/typespec%400.57.0) has introduced the concept of **values**, which can be defined in the same way as types, and can then be used as defaults and in other places. The grammar should implement this and apply the correct highlights and indents. I'll probably get around to this in a week or 2, in the meantime I'm open to help anyone who wants to take a stab at this.