This addresses the following issues:
* The JSON Schema spec doesn't actually define whether objects with
a "properties" key still require `"type": "object"` or not.
VS Code for instance largely pretends as if it's implied, but when it
encounters them inside a `oneOf` tree, then it behaves as if it isn't.
In other words, we need to always set `"type": "object"`.
* Declaring an `oneOf` containing a `"type": "string"` and an `enum`
doesn't work, because if one of the `enum` cases is given, it results
in both variants to match, since any `enum` is also a `string`.
We have to use `anyOf` instead.
* `SuggestionSource` used `"BuiltinSuggestionSource"` inside a `type`
key which doesn't work. We have to use `$ref` for that.
Closes#13387
## Validation Steps Performed
* VS Code stops complaining ✅
* https://www.jsonschemavalidator.net/✅
0b9f041706