fix: Add another `defineItem` signature when `init` function is passed

[toofishes]

Overview

https://github.com/wxt-dev/wxt/pull/1601 adjusted the typing of defineItem to examine whether defaultValue or fallback were passed, and only then, remove the null from the possible returned value. However, it did not accommodate the case where an init function is passed to prevent a null return, which was previously allowed.

Add a new type signature for this case, and add some tests for type expectations to catch the issue.

Manual Testing

Added type tests that fail before this change, and succeed after.

Related Issue

• #1601

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[RayGuo-ergou]

just about to create a PR for this :) haha thank you!

[aiktb]

LOL I didn't see this, I created a similar one

[toofishes]

@aklinker1 Is there anything I can do to help get this reviewed and merged?