…/quoted

# Description

## Problem\*

Resolves #5285 

## Summary\*

Implements `as_type`, `generics`, and `fields` on the `TypeDefinition`
type.

The `Type` type isn't really supported yet so `as_type` and `fields`
return `Quoted` token streams instead of `Type`s for now.

## Additional Context

A few bugs still need to be fixed after this PR for the vertical slice
to work.

## Documentation\*

Check one:
- [ ] No documentation needed.
- [ ] Documentation included in this PR.
- [x] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

---------

Co-authored-by: Tom French <[email protected]>