Type-check lintRule() implementation

[jablko]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)
• If applicable, I’ve added docs and tests

Description of changes

What do you think about narrowing the coerce() return type from unknown to a type parameter, which is less permissive?

What I did was:

• Change the return type from [Severity, ...Array<unknown>] to [Severity, Options|undefined]
• Change the parameter type from unknown to Options|[boolean|Label|Severity, (Options|undefined)?]
• Include the Options type parameter in the Rule type

[codecov-commenter]

Codecov Report

Merging #290 (04911b7) into main (29bb3b7) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #290   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           73        73           
  Lines         8422      8428    +6     
=========================================
+ Hits          8422      8428    +6     

Impacted Files	Coverage Δ
packages/unified-lint-rule/lib/index.js	100.00% <100.00%> (ø)

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