fix(signal-slice): Do not allow optional properties in signalSlice

[palexcast]

Optional properties in the initial state pose an issue with selectors, this PR forces a user to set it as undefined instead.

With ?:

With | undefined

Issue explaining in detail: #176

[joshuamorony]

Thanks for this @palexcast - I'm happy with the change, and I don't know how else it would be fixed anyway (although @nartc has dealt with most of the tricker typing situations for signalSlice so he might have thoughts). I think we can just add a brief note about this to the docs under Basic Usage to explain that it is not allowed (we probably don't need to get into the specifics right away).

[nartc]

@all-contributors please add @palexcast for code

[allcontributors]

@nartc

I've put up a pull request to add @palexcast! 🎉

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[nartc]

LGTM. Can you add some comment to the ts-expect-error directive?

[palexcast]

LGTM. Can you add some comment to the ts-expect-error directive?

Of course, done 👍