refactor: change the signal setter type to show clearer type errors

[otonashixav]

The effects of this change are viewable here: https://tsplay.dev/WkMnDN

[coveralls]

Pull Request Test Coverage Report for Build 1988634838

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 89.275%

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Totals
Change from base Build 1950433993:	0.02%
Covered Lines:	1211
Relevant Lines:	1294

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💛 - Coveralls