[TypeKit] $.value.* APIs

### Clear and concise description of the problem

To support operations on values, which is at least create, but perhaps things like `isAssignableTo(Type)` could be useful as well.

### 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.