Thanks for posting this question. Fields are specific to Custom Types, so you can't order by fields in the API.
Alternatively, I would suggest a workaround:
You can query multiple Custom Types with the any predicate. Then, in your code, you can sort the results with Array.sort().
The downside is that, if you have more than 100 matching documents, you'll need to perform a recursive query to fetch them all. However, v6 of @prismicio/client is currently in alpha, and it has a getAll method, which performs a recursive function. So, you could do something like:
@pculley We'll probably move it to beta in the next few days, but we don't have an ETA for stable. It's already been tested, so the beta period will be more so for communications, updating documentation, and gathering feedback on features. I hope that helps.