Generate defaults with Partial<T>

[sdboyer]

Currently, the defaults we generate may not pass Typescript's type checker. An incomplete cue.Value (one that has at least one required field without a default and therefore cannot be rendered to a concrete form like JSON) will fail, because the default const produced will lack any entry for that required field.

I think the simplest fix here is just to always generate our default consts with Partial<T>. We could do so conditionally based on a concreteness check, but until we're aware of a use case (@ryantxu?) that really requires a default not being Partial, this seems the best solution.