Make untracked return value & effect handle teardown

[dy]

Hi @WebReflection

I know you mentioned there's no fancy features in this library, but I took liberty to propose 2 updates.

1. untracked function seemingly must return a value - I missed that out making #5
2. effect callback can return a teardown (like useEffect) - not directly documented in preact, but very handy feature, low-hanging fruit. It's minor breakage of existing API, but in favor of more standardization.

Please let me know if you find value in these changes.

[coveralls]

Pull Request Test Coverage Report for Build 8069503242

Details

• 0 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 8050013448:	0.0%
Covered Lines:	149
Relevant Lines:	149

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💛 - Coveralls

[WebReflection]

done?

[dy]

Yep, looks perfect, thanks @WebReflection