Improve DeepPartial using recursive types

[MatthiasKunnen]

Description of change

Fixes #2904.

This change improves the DeepPartial check and prevents TypeScript from errorring unnecessarily in cases described in the aforementioned issue. On the flipside, it now errors on .save() in case a string property is passed instead of a Date. As such this PR will be a breaking change. I created a second commit that highlights what resolving the breaking change would look like.

What are your thoughts?

Pull-Request Checklist

• Code is up-to-date with the master branch
• npm run lint passes with this change
• npm run test passes with this change
• This pull request links relevant issues as Fixes #0000
• There are new or updated unit tests validating the change
• Documentation has been updated to reflect this change
• The new commits follow conventions explained in CONTRIBUTING.md

[imnotjames]

There's still a few more type issues here. Are you going to be cleaning those up?

[MatthiasKunnen]

Yes, but only if the maintainers are interested in the PR. If they are not interested, then there is not much point to fixing them. I'm waiting for feedback atm.

[Alwinator]

@imnotjames @MatthiasKunnen @pleerock Any news here? I would need this feature!

[pleerock]

it's postponed since its a breaking change. No estimates for now.

[pleerock]

I applied more simple change based on this SO answer. Your test seem to pass and change doesn't look breaking. I think we can merge it.