[FEATURE] improve types to not allow random function return properties to be provided

[filmaj]

This happened when a user inadvertently provided return { complete: false } (rather than the correct { completed: false } - a missing d caused a lot of confusion!).

In the above, while there is an autocomplete experience guiding the user towards specifying the correct property completed with a d, it is still possible for the user to e.g. not notice or otherwise typo the property.

A fix here should add a type test that ensures providing invalid / typo'd properties (such as complete vs. completed) raises a TypeScript error.