Fix getFieldset with interfaces and nullable schemas

[aaronadamsCA]

This should fix #499, specifically the types failing to reveal getFieldset in these two scenarios:

• Your FieldSchema is an interface, not a type.
• Your defaultValue is nullable.

Known existing issues this PR doesn't try to fix:

• getFieldset is incorrectly available on other object types, e.g. Date.

I'm filing this as a draft, as I have no idea what kinds of tests to write for this.

[aaronadamsCA]

Ah, okay, I see now that these are tested via a playground package type check, so now I know what to run locally to test changes.

I fixed a very weird problem caused by TypeScript quirks:

• NonNullable is implemented as T & {}, since {} means "any non-nullish value".
  • Therefore NonNullable<unknown> resolves to {}
• But {} extends object resolves to true, probably for legacy reasons.
  • Therefore NonNullable<unknown> extends object resolves to true (yikes!).

So I stuck with Exclude, which works just fine.

I also updated the tests to use interface instead of type. They fail without this change and pass with it.

[edmundhung]

Thank you!